I grabbed Vanessa’s waist with both hands and slammed my hips into her heart-shaped butt, forcing the air from her lungs and making her squeal out loud. She twisted her head around so she could look at me as she gaspingly spurred me on to fuck her even harder. Her pussy was as tight and wet as ever and the pressure in my balls was rising fast. Luckily my friend was also getting close again, rubbing her clit like a maniac and moaning ever louder with each of my thrusts.

Vanessa had already told me it wasn’t her safe period, so I knew I had to pull out soon, but she reminded me all the same.

“Don’t forget to pull out before you cum, you can’t come inside me today.” she groaned as she grabbed the pillow and pushed her butt firmly back at me.

“Will you suck me off then?” I asked, even though I knew she wouldn’t. Vanessa wasn’t like her friend Susie, who would gladly make me cum with her mouth whenever I couldn’t finish inside her. However, Susie was in a committed relationship right now, so she wasn’t as often available for a casual fuck as she used to be.

“No way!” Vanessa grunted while another orgasm hit her, turning her protest into an unintelligible moan. Never have I fucked a girl that could cum as easily or as frequently as she.

“At least let me cum on your tits if I can’t cum in your pussy,” I groaned while I kept pumping her quivering cunt.

“Do you have to? Why can’t you just use a tissue?” she panted, barely concealing her annoyance that she had to talk when she wanted to cum.

“Come on, it will make your tits grow big.” I pressed on.

This caught her attention.

“It does?” she asked excited.

Vanessa was a cute girl with a petite 5’3” body with all the curves in the right place. Her firm breasts weren’t exactly small, but a little bigger would look great on her. I knew she thought so too and it was too easy not to exploit that knowledge.

“Of course it does, everyone knows that.” I added.

She looked at me for a second, trying to think hard while I kept pounding her pussy without mercy.

“Okay, you can cum on my tits, but don’t hit my face!”

“Thanks. Now get on your back, I’m not done yet.”

Vanessa rolled over and spread her legs again. Her cute pussy opened wide, like a pink butterfly spreading its satiny wings. She always kept it cleanly shaved, something I deeply appreciated, as I loved to eat a girl. But right now, as tempting as her moist pink pussy may look, more than anything else I wanted to sink my cock back into it and keep fucking until my balls were completely drained.

“Put it back Tom, I need your big cock back inside me!” Vanessa begged as she slowly rubbed her pussy. Goddamn! How horny could a girl be?

I crawled between her legs and nuzzled my cock between the silky lips. I lifted her legs and pushed my hips forward, squeezing myself back inside. For such a slut, she had an incredibly tight pussy.

A content smile appeared on Vanessa’s face as I pushed my cock all the way home and started fucking again. The pressure in my balls had receded a little while we switched positions, so I knew I’d be able to pump her a little longer before the inevitable would happen. Soon, my classmate was moaning again and lifting her butt off the mattress to meet my thrusts.

I grabbed her leg more firmly and slammed my hips at her without mercy. Vanessa was frantically kneading her breasts, as if she was priming them for their medicine. The view and feel was second to none and I knew I wasn’t going to last much longer. With my final series of thrusts I managed to give her one last, massive orgasm.

Vanessa’s body went rigid as she raised her hips as high as she could, taking me in all the way and rubbed her clit to intensify and prolong her orgasmic bliss. Her boiling pussy squeezed and sucked, milking the cum from my balls.

“I’m gonna cum!” I grunted

“Quick, on my tits, make ‘em big.” she gasped between moans.

I groaned as I pulled out just as my orgasm hit. I grabbed my cock and stroked it as big jets of cum splashed on her perky tits. Six, seven more streams I managed to shoot, covering Vanessa’s breasts with streaks of white.

She kept rubbing her clit while I sprayed her and within seconds she climaxed again, just as powerfully as the first time. Her firm tits trembled and jiggled as new waves of spasms raced up and down her body until we both slumped down on the mattress.

Finally drained, I rolled off the girl and watched her wipe up every stray drop of sperm from her belly and rub it into the smooth, creamy skin of her breasts.

“I hope mom won’t smell all this cum on me,” Vanessa muttered as she got out the bed, “or I’ll be grounded again.”

She rummaged through her bag and retrieved a small bottle of perfume and sprayed a little over her entire body. After that, she gathered her clothes and got dressed, kissed me on the cheek and walked to the door.

“Will I see you again on Monday?” she asked, “I should be safe then ... I’ll let you cum inside me. Or you can come on my tits again.”

“Sure.” I sighed and she left, leaving nothing but the heavy scent of her perfume and a pleasant drained feeling in my balls. These girls were killing me, but I loved it.

As Vanessa walked out my room, my sister Jenny walked in. I wasn’t surprised; I knew she had been watching us. She always was.

My sister liked girls, and more than anything she loved to see a pretty girl cum. Given the way Vanessa had been wailing, there was no chance Jenny could not have known what was going on in my room. She had probably been standing in the hallway, looking in through the half open door, one hand in her pants, rubbing her pussy as she watched my friend shake and convulse in orgasmic pleasure.

My sisters and I had a little agreement. Jenny and Emily didn’t tell our parents about the girls I brought home to fuck and I didn’t tell them about the twins fingering and licking each other’s twat when they were alone at night.

Jenny greeted my friend and watched her walk out the house, shaking her head. “That girl is going to be a mommy soon, if she doesn’t learn to be more careful.” Jenny said as she sat down on the bed.

“Yeah I know, but she’s a terrific fuck.” I sighed as I searched the floor for my clothes.

“Hmm, I could see that. I bet her pussy tastes as sweet as honey.”

“Well, it doesn’t,” I sighed. “But it’s worth it.”

“Ugh! You really are a pig,” Jenny giggled “you wouldn’t know a nice pussy from a hamburger if your life depended on it.” she said as she picked up my shorts.

“At least she smells nice.” she said.

“Nice? My room reeks like a French whorehouse! How am I going to explain this to mom?”

“If she asks, just say that I did it.” she said. I knew her sacrifice wasn’t completely altruistic. If mom ever found out what went on in my room while she was at work, there would be no more female company for me, and therefore no naked, writhing girls for Jenny to masturbate to.

“Thanks, I might do that. But I’ll get some fresh air in, just to be sure.”

“Don’t you think you’d better cover that up, first?” she said as she pointed at my cock, still swollen and sticky with pussy juice. “Before you expose yourself to all the neighbors?”

“Don’t worry; I’ll keep the curtains closed. I think I’ll shower first anyway.”

I took a quick shower to wash off the sweat and Vanessa’s juices, dried off and wrapped the towel around my waist. As I stepped back into my room, Jenny was still sitting on my bed.

“You know that was total bullshit don’t you?” she said.

“What was total bullshit?” I asked.

“What you said, about sperm making tits grow. It’s just an urban myth.”

“It is not, everyone knows it’s true.” I repeated the claim I staked earlier. Of course I knew it was bull, but I wasn’t going to let my sister call me a liar. Besides, how much does a lesbian know about sperm anyway?

Jenny wasn’t as gullible as my friend and called my bluff.

“I bet you $50 it doesn’t work.”

“Okay,” I accepted “and how do you want to prove who’s right?”

“Don’t worry about that, I know just what to do. You’ll like this.” She said with a slight smile.

As I was waiting for her to unfold her plan, Jenny started to unbutton her blouse and bared her beautiful breasts.

“I’ll let you cum on my left breast, but only the left.” she said, cupping the left magnificent orb in one hand. “It’s slightly smaller than the right one.” she added as she grabbed the other one too.

“They’re both beautiful.” I sighed. Honestly, they were.

“Thanks.” she smiled “Now, I’ll let you cum on this one for a whole month, and if it’s still smaller than the right, you owe me $50.”

“And if it grows?”

“I’ll give you the $50 and you can cum on both of them until they’re as big as mom’s. It’s a win-win situation.”

It wasn’t difficult to decide. Although I fucked Vanessa and Susie at least once or twice a week, as well as the occasional one night stand, I still jacked off almost every day. Doing it on Jenny’s gorgeous tits would be a bonus, even if I was sure to lose the bet.

“You’ve got yourself a deal, sis. And you know what?”

“Well?”

“I’m getting hard already. Are you ready for your first treatment?”

“Wait, let me take off my blouse first.”

I grabbed my cock and started stroking it. My sister had seen me naked countless times and watched me fuck and cum so often, I didn’t feel any embarrassment to do this in front of her. Jenny moaned softly as I jacked my cock and if I hadn’t known for a fact that she was gay, I would say she was getting quite aroused too.

It truly was a shame that my sister wasn’t interested in boys, because she sure did have magnificent tits. Though they were a bit small, they were perfectly round and very firm. Her immaculate skin was soft and creamy and she had little pink nipples, which were all puckered up as they stood erect. The fact that these beautiful globes belonged to my lesbian sister only heightened my arousal, and even though I had deposited a huge load on Vanessa’s tits only ten minutes ago, I already felt my muscles tighten as another orgasm rose. Moments later I felt the first wave of sperm rush up my cock and shot a thick white stream of cum on Jenny’s left boob.

My sister watched in awe as my seed sprayed over her tit. After I was done and had wiped the last drops on her supple flesh, Jenny rubbed my sperm into the creamy skin and kept massaging her breast until every last drop was absorbed.

“There, it’s all gone. If it doesn’t work now, it never will. Agree?”

“Whatever.” I panted, exhausted after another great release.

My sister just laughed at me and told me to come to her room again that night.

For the next month, I would come to Jenny’s room every morning and every night. She would take off her nightgown and, wearing nothing but her little white panties, present her left breast, ready to receive another load.

As my sisters shared a room, this meant that Emily would be there too. It was pointless to keep it secret from her, and she didn’t seem to mind. She didn’t join us, either. She never touched me or her sister, not even herself. She just lay there, silently, never letting her eyes stray from my cock or Jenny’s breasts.

The days flew by as I generously coated my sister’s left breast with copious amounts of cum. Much too soon, an entire month had passed and it was time to see if our experiment had yielded any result. We both watched as Emily carefully and meticulously inspected and felt her twin’s breasts. After a minute she finally concluded that the left breast was still slightly smaller than the right one.

“There, I told you it was bullshit.” Jenny said, a little disappointed she hadn’t grown a bit, but happy with the 50 dollar she won.

I did not complain. Sure, I had worked hard to get that money, but it was well spent. 50 bucks was a small price to pay for a month of playing with my pretty sister’s perfect titties.

As Jenny was buttoning up her blouse, I couldn’t help but wonder if, with the conclusion of our experiment, her breasts were now forever off-limits for me. It would truly be a loss for mankind.

“Did you say something?” Jenny said suddenly. I looked up, to see who she was talking too. It turned out she was addressing our sister.

“No ... I ... didn’t say anything.” Emily stammered, almost whispering. She had a quaint expression on her face, as if she was scared or about to cry.

“I’m sure you were going to say something.” Jenny pressed on. “What is it?”

There was clearly something on her mind, and though she might not want to, Emily had to speak up.

“It’s not bullshit,” she blurted out, her cheeks all flushed. “Your breasts just haven’t grown because you’ve been doing it all wrong. I could have told you it wouldn’t work that way.”

I looked at Emily in disbelief. For weeks she had known about our wager and hadn’t said a single thing. Only now, after we were finished, did she tell us we did it wrong.

“What are you talking about?” Jenny asked.

“It doesn’t work like that.” Emily explained “The way you were rubbing his sperm in, it doesn’t work. You have to take it ... in ... You know, swallow it.”

“Eew!” Jenny exclaimed with disgust. “You’ve got to be joking.”

“No it’s true, honestly.” Emily said softly, her eyes averted.

“I still don’t believe it. Prove it first, and then I’ll believe you.”

“You can’t prove it like that.” Emily said. “You can’t just swallow cum for only one tit; they’d both grow at the same time.”

Jenny looked at her sister for a minute, a mischievous smile on her lips.

“You know that’s not a problem.” she said mysteriously.

Emily gave Jenny a blank stare and asked what she meant.

“Well, your tits are just as big as mine. So, if you ... swallow ... his cum for a month and your tits have grown more than mine, we have proven it works. I’ll admit that you’re right and let you have this 50 dollar bill. Deal?” Jenny said holding up the banknote I just gave her.

I realized my sister had just volunteered my services without even bothering to ask for my consent. I guess she assumed I would go along anyway. She was right.

“Why would I have to swallow it? It’s you who started this bet!” Emily protested, rather weakly.

“Gross! I don’t want big breasts badly enough to do that! Besides, I said it didn’t work. It was you who said it did, so it should be you who has to prove it.”

Jenny’s logic was obviously flawed, but Emily said nothing and only shook her head, barely perceptible.

“I know you want to, Emmy.” Jenny whispered, deliberately loud enough for me to hear.

Emily flashed an angry look at her sister. It was only a fraction of a second, but I saw it and I knew what it meant. There was something between the two, something Emily wanted to keep secret.

“Don’t act like you don’t know what I’m talking about.” Jenny said.

“You can’t!” Emily hissed and threw another angry look. “I never...”

“I know you’re lying,” Jenny continued, ignoring her sister’s pleas. “You’ve been all over me every time Tom left our room, knowing I was covered in cum. You’ve been dying to taste it right from the source, haven’t you?”

“No.” she whispered, looking away. Emily was a poor liar, and she knew it too. She might as well have said yes.

While my sisters looked almost identical, their personalities were almost opposite. Although they could be highly competitive, Jenny would inevitably take the lead the lead and the more demure Emily would always follow. But, while I knew Jenny preferred girls over boys, I never could figure out my other sister.

As far as I knew, Emily never dated anyone, boy nor girl. I just assumed that having her twin as a lover was enough to keep her satisfied, so maybe she was a lesbian too. It was a bit of a surprise to learn she actually did have an interest in boys, even if she wanted to keep it secret. Why she wanted that, was another mystery.

Undeterred by her reluctance, Jenny pressed on, forcing her twin to react. “Come on Emmy, I only want to help you. I know you’ve been curious about boys for some time. It’s only natural ... for most.”

Emily didn’t answer, but stared at her feet and nodded.

“So, do we have a bet?” jenny asked as she knelt next to me.

“Yes.” Emily whispered.

Even before Emily had answered, Jenny had already undone my fly and now she pulled down my pants and underwear. My cock had never gotten this hard this fast. In a fraction of a second it was standing like a flagpole, throbbing and ready for action.

“Get on your knees sis, and open your mouth.” Jenny ordered.

As directed by our sister, Emily got on her knees in front of me, her face only inches from my groin. Still a little shy, she closed her eyes and opened her mouth. Her slightly extended tongue was a perfect target for me to aim at.

I grabbed my cock and started stroking it. In less than a minute I felt my sperm rush up my penis and watched it land on Emily’s pink tongue. Jet after jet splashed in her mouth, gathering in a small white puddle just behind her teeth.

Only after she was convinced I had given her all I could, my sister closed her mouth and swallowed. Then she opened her mouth again to show us she drank every drop.

Then, she seemed to realize what she just had done, and Emily was once again her demure self. Embarrassed, she threw herself onto the bed, burying her face beneath the pillows.

“Well? Was it everything you hoped for?” Jenny teased as she pulled away the pillows. “Can you feel your breasts growing?”

This caused her sister to chuckle sheepishly. I could see the relief on Jenny’s face. Although her twin was still unable to talk, and her cheeks were deeply flushed, she wasn’t freaked out too much.