A Guide to Writing Interactive Erotica/Facts
There are several rumors and misunderstandings about sex that can happen, and these rumors really like to breed in the pages of erotic writing where untrue information can be treated as true inside of an erotic fiction. It is very possible for your story to work out just fine with inaccurate information, it often won't even break immersion all that much, especially for readers who don't actually know any better. However, it can often actually be more interesting and engaging for a reader when accurate information is used.
Semen
Semen is the collective name for all components of male ejaculate. This actually includes 3 components in approximately equal proportion. An alkaline fluid meant to neutralize harmful acidic fluids in the woman's vagina produced by the prostate, a neutrient fluid released by special glands on the back of the prostate called ejaculatory glands (named for the ejaculatory ducts which inject the neutrient fluid into the urethra) which serve to nourish the sperm in order to allow them to last longer as they seek the woman's egg, and of course the sperm itself which comes from the testicles. Pre-cum is not technically a part of semen, but can be counted as well as a 4th component.
No sperm in pre-cum
Pre-cum does not contain any sperm cells. It is a lubricating fluid released from a gland at the root of the penis. Meanwhile, sperm must travel over twice the distance and get past the prostate before entering the penis. The purpose of pre-cum is both to provide fluid to lubricate entry during sex, and also to neutralize the acidic nature of urine in the urethra which is harmful to sperm cells.
One likely origin of this myth is that when the pulling out method is used to prevent pregnancy there are times where it is used faithfully but the woman still gets pregnant. This is usually caused by cases in which a man has sex/masturbates and manages to ejaculate once, and then sticks it into the vagina afterward. In these cases, the pre-cum will force out sperm that is still trapped in the urethra. This means that cases in which a woman sucks a man off or other forms of foreplay are engaged in that cause the man to ejaculate his first load outside the woman before moving on to vaginal insertion can cause the woman to become pregnant even if the man does not finish inside her for the second round. The fact that this method of foreplay was used means that the residual semen trapped in the urethra can get into her vagina.
(IRL tip, this issue can be mitigated by having the man urinate between ejaculations before he sticks it in.)
Seminal fluid is yellow
As any man who ejaculates into tissues would know, the tissues get a yellow tint when the semen dries. This is not due to any kind of residual urine or anything of the sort, this pale yellow color is actually the color of the nutrient fluid produced by the ejaculatory glands. The sperm cells themselves are white, but as they die off in the tissue the yellow color of the seminal fluid itself begins to show through.
Hymen
The hymen is a thin skin membrane that is inside a girl's vagina and covers it. The purpose of the hymen is most likely an adaptation from ancient times when children were naked and would often sit on the ground, the hymen would be very useful to prevent infection. In the modern day where babies wear diapers, this function becomes even more useful and important for baby girls.
Hymen is not destroyed in sex
During sex, or other insertion of an object into the vagina, the hymen is not torn away or completely destroyed. The hymen has a small hole in the center of it in order to allow for mensturation to pass through it, and if an object is inserted into the vagina the most typical result is for the hymen to distend inward and for that hole to stretch. Capilaries are broken in this process, which produces the bleeding often associated with "loosing" the hymen. However, the hymen is still present. It is only stretched. Most of the residual ache associated with a girl "loosing" their virginity is a result of the hymen attempting to recover from this extreme stretching.
The hymen can actually tear as well. This results in a lot more bleeding than the stretching result, and the bleeding can continue for far longer.
The only ways to completely destroy the hymen such that it will never recover are either to have it surgically cut and removed, or for an extremely large object that is far larger than the typical male penis to pass through the vagina. (A baby's head most certainly fills the latter criteria.) So long as one of these two scenarios have not occurred, there will always be some residual vestiges of the hymen.
Insertion is not needed to remove the hymen
As covered above, the hymen is not destroyed through sex, it is only stretched. There are methods other than insertion that can stretch the hymen. Many people have heard that a physically active girl usually does not have a hymen despite never having had sex. This is not a result of anything breaking the hymen through insertion either. The truth of the matter is that active girls have probably done the splits or something similar several times. The splits or similar such activities can actually stretch the hymen without an object ever having been inserted. It does not even have to be the lateral splits, just running with a long stride can do it if this activity is repeated several times a day. The gradual stretching will add up and it will result in the hymen stretching far enough as to be unnoticeable during that girl's first sexual encounter.
Due to this, almost every girl involved in gymnastics, sports, ballet, or cheer-leading is likely to not have a hymen.
Other contributors
If there is another medical level or birth control related fact about sex you might want to share, please add it here.