It was the Fourth of July and just starting to get dark. I'd spent the day with my live-in girlfriend, Mary, and her family at the Riverfest, sitting on a blanket and listening to the music coming from the bandstand, eating grilled hamburgers and hot dogs and drinking the overpriced beer that the vendors sold. There was a pretty good crowd at Centennial Park, and the weather had cooperated nicely for a change, with none of the usual afternoon thunderstorms that our area is known for in the summer. Even the mosquitoes weren't too bad. Mary's folks had their own blanket and her younger sister, Alexa, shared another one with her boyfriend, "Horse."

Well, actually, his real name was Bob Wessel, but he picked up the nickname in high school. I figured at first that it was a bad play on the name of the Nazi thug that the Hitler Youth song was named after (Horst Wessel), but then he ended up in my gym class during my junior year. Now, I'm not a guy who goes around looking at other guy's dicks in the locker room, but Horse was hung like a... well, a horse. Seven inches soft, easily. After a while, I guess he wore the nickname with pride.

Mary's twenty, a year younger than I am. She's a pretty blonde with big blue eyes that give a false impression of innocence. She's a little shorter than I am, about five feet, six inches tall, with a slender waist, large breasts and deliciously curvy hips. She's also the most sensual and most unabashedly sexual woman I have ever been with. She's a firecracker in bed, and she is proud of it. She doesn't like condoms, so when we moved in together, we got all the tests and when they came back negative, we stopped using them. She's shared her fantasies with me, about threesomes and moresomes, among other things. Today, she'd dressed patriotically for the Fourth, wearing a red-and-white striped polo shirt and a short blue denim skirt.

Alexa had just turned eighteen a few months back, and she was hot. She is taller and thinner than Mary, with dark hair and green eyes. I'd noticed how pretty she was for a long time, but she WAS underage and she WAS my girlfriend's sister. I'd had sexual fantasies about her but it wasn't anything I was going to tell Mary about; that would be about the quickest way to turn her into an EX-girlfriend.

The Riverfest has the best fireworks display in town, which is the main reason I like it. It usually runs about a half hour or so. They shoot the fireworks off over the river, so if they misfire, they'll just end up in the water. Centennial Park is along the south bank of the river, by the downtown business district. To the east of the park is the Yacht Basin.

Mary was snuggling up behind me. We've been together for over a year and living together for six months, so I've gotten to know her little signals. When she's horny and wants some loving, she'll snuggle up behind me with her erect nipples poking me in the back. Well, it felt like there were a couple of spear-points boring into my back. I looked back over my shoulder at her and saw the wicked grin on her face.

"Mary," I whispered, "The fireworks are about to start! Can't this wait until we get home?"

"Nope," she said.

"Well, we can't do it HERE," I told her. "There's hundreds of people around! Even if we go back in the bushes..."

"Don't be a spoilsport!," she interrupted, "Grab the blanket and follow me. I know just the spot."

I knew that there was no reasoning with her. I just hoped she knew of a spot where the cops wouldn't nail us for indecent exposure or lewd and lascivious conduct. She took my hand and we made our way along the edge of the crowd, heading east toward the Yacht Basin. Darkness was settling in and I knew that the fireworks would be starting in a few minutes.

She led me past the Yacht Basin toward a little shed on the far side. There was one bulb dimly lighting the west side of the shed, but it was dark on the other side. We started to walk around the shed to the dark side when we heard the unmistakable sounds of passion around the corner.

"Shit!" Mary grumbled under her breath, "Someone's in MY spot!"

We peeked around the corner and I was surprised to see by the dim light that it was Alexa and her boyfriend. Horse was standing with his back to the wall, with Alexa kneeling in front of him, giving him head. She had about half of his dick in her mouth, and there were still a few inches that wouldn't fit. I heard a gasp next to me from Mary.

"Jesus!" she hissed. "She told me he was big, but not THAT big!" Behind us, we heard the tell-tale whiz of the first rocket of the night being fired and a moment later, a bright explosion of white sparks illuminated the night sky, followed by a thundering report. In the brilliant light, I saw Horse look in our direction and freeze. We were spotted!

Mary strode over to her sister, who was getting to her feet, and pulled her a few feet away from Horse. They talked for a minute or so as the fireworks continued to explode in the sky. I saw Mary looking at Horse's cock with unconcealed lust, and I saw Alexa look at me with a wicked smile. Mary came back over to me as Alexa walked to Horse and started whispering to him.