You might call this a "primer." It is meant to educate the ignorant ... primarily the authors of the stories you read on this website.
If there is one thing I cannot tolerate, it is inaccuracy in story writing. When I read a story, no matter how entertaining or well composed it is ... if it is factually inaccurate, it destroys the story for me. With that said, let me tell you a few factual truths about incest and sex.
1. The uterus or womb of a female is never entered by the penis. Never! The OS (cervical opening) is a tightly restricted muscle that is made to prevent that. When I read a story about someone who pushed so hard that they penetrated the womb, it makes me cringe. Just to insert a tiny IUD requires that I hold the OS tightly with tenaculums during the procedure, and an IUD introducer is a thin tube, just barely larger than an ink pen refill. There is no way in which anyone could ever enter the uterus with their penis. Not only is the opening too small, but it lies perpendicular to the vaginal canal and fairly flexible. Men with overly large penis can cause females some discomfort during intercourse if repeated "bumping" of the uterus occurs.
2. No one ejaculates directly into the womb. Chances are it would kill the female. Semen is highly toxic to the uterus. The proteolytic enzymes in semen can cause severe anaphylaxis if it is introduced into a woman's uterus, resulting in shock and death. Early attempts at artificial insemination proved just how dangerous it can be. Luckily, sperm that enter the uterus have none of these allergens in their makeup, thus allowing them to swim freely and happily to their inevitable demise. So you authors who write about "flooding" a girl's womb with their seed ... balderdash. What you probably are describing is the vaginal fornix or the recto uterine pouch.
3. Incest among teens is a definite bell curve. That is it occurs most commonly in teens about the age of 16-17 years of age. The bell tapers off quickly in both directions. Less than 20% of all incest takes place during the 12-15 and 18-20 year groups. 75% of all incest occurs during the peak years mentioned. The last 5% is randomly distributed throughout the participant's ages. Note that when I say incest, I am talking about purposeful contact for sexual gratification. I am not including experimental or exploratory sexual encounters. "Show me yours and I'll show you mine" behavior is commonplace and is not incest unless pursued beyond simple touching and visualization.
4. 70% of all incest occurs between brothers and sisters. 20% occurs between fathers and daughters. Incest between cousins is no longer considered incest by most reporting agencies. Most states allow marriages between cousins. Mother/son incest is extremely rare. Homosexual incest is more prevalent between sisters and fairly rare between brothers, and statistically insignificant in all cases. Nephew/aunt incest is also extremely rare. Uncle/niece incest occurs at a rate similar to fathers and daughters, but since uncles may not be related to their nieces by blood, statistics do not discriminate, and incest is actually often misclassified as statutory rape and not reported as incest.
5. Mutual masturbation by brothers and sisters is not considered incest by most agencies unless actual intercourse takes place. Again this occurs at a higher frequency than reported as many brothers and sisters explore themselves during adolescent growth. The standards for labeling a relationship as incest varies in both state and country. The legal definition of incest varies from state to state. Technically, any contact between genetically close relatives that has the main purpose of sexual gratification may be considered incest, whether intercourse occurs or not.
6. Orgasm may occur at any age; however, the degree varies greatly and directly with neurological development. It is not uncommon for parents to find their prepubescent 5 year old daughters rubbing a pillow against their genitals. However, it is fairly rare to find the same behavior in males at that age. Stimulus to the genitals in young children is more likely to stimulate a "tickling" sensation rather than a sexually gratifying feeling. It isn't until puberty that the nerves mature enough to reach the different pleasure centers of the brain. This is not to say that a sexual orgasm in impossible in children, just that it is very unlikely.