“Who says you can't love something that's small and finishes fast?”
Sexual physics concerns the application of physics to a receptive female. The relevance to creating relatives is well-known; but less so the relevance of Relativity. Young males tend to have sex at very high speeds for very short durations. Albert Einstein's work warns us that sex approaching the speed of light (referred to as c, with an obvious double meaning to the "c-word") carries a variety of risks, many of which make AIDS seem like a mere roll in the hay.
Long before approaching c, the speed of sex would exceed b, the speed of sound. This leads to B — an enormous sonic boom, inducing your parents to come upstairs to find out what the Hell is going on. You would be hard put to explain that the two of you were merely using your Junior Chemistry Set, having disrobed to avoid altering the pH. Some females seem to enjoy loud noises during sex, but if the partners ignore the risks and continue, the neighbors will eventually call the police.
As objects approach the speed of light, their length contracts. A penis, whose average length is 13cm, will at 0.99c contract to a length of only 1.8cm. This is a proportion of positively Japanese proportions. In theory, two things could happen: Either the loss of penile length would reduce the speed of sex and comprise a "feedback loop," or the male would continue to accelerate and, at orgasm, would have a zero-length penis, which doesn't do anyone much good.
Another phenomenon as velocity approaches c is that time itself dilates; that is, everything outside the passing member passes time at a faster rate during this pastime. As everyone knows whose pastime is instead to ridicule Star Trek, sex approaching c would tend to make the partner age rapidly, as well as the rest of the universe. Long before one could climax, one would be committing necrophilia.
Relativity also predicts that objects acquire additional mass as they approach the speed of light. The penis would essentially become a black hole. Instead of the vagina capturing the penis, the penis would capture the entire partner, and probably both partners and even the bed. Now, some women seem to like sex with blacks, and some men are repeat patrons of the so-called black holes downtown. Nevertheless, too-rapid sex could cause a feeling of loss. John Wayne Bobbitt felt it, when he ceased to feel it, from the equally risky practice of sex with Ecuadorians. Happily, Ecuadorians are notoriously bad at hiding things, and Bobbitt was sewn back together.
Friction increases at high speeds. "Sexual friction" is a chronic problem, and the friction of sex near the speed of light is not even mitigated with KY jelly. As much as the woman may say she wants "hot sex," she does not want it this hot, probably. Sex at the speed of light risks ignition, explosion, and vaporization. The condom, of course, would first liquefy.
One imagines that ejaculation would also be harmful to the partner, because the speed of the fluid would be added to the speed of the penis. However, as one approaches the speed of light, velocities are not additive, as of course 0.75c + 0.75c ≠ 1.50c. A hypothetical observer standing on the head of the penis and looking up would not see ejaculation at all but merely get his hypothetical feet wet, and ironically, only when wearing rubbers.
Excessive energy use
The amount of energy required to accelerate the average penis to 0.99c is approximately 16∙1015 for each thrust. This is equivalent to the annual use of energy by the entire city of Arequipa, Peru, or the amount of food energy of 78 trillion Weetabix, which is substantially more than the Recommended Daily Intake, even though the average adolescent would not want to interrupt sex to have several billion breakfasts.
Lack of visual appeal
None of the above physical effects of high-speed sex translate well onto the silver screen. In addition, movie critics pan high-speed sex for being "over too soon," despite modern advances in cameras that capture more than 24 frames per second. Their use in The Hobbit was successful only because all the adolescent sex was cut from that feature film. The porn industry cannot make this appealing, even with fake eyelashes and shaving of the private parts. This is why the industry does not use teenagers but instead washed-out actors and actresses in their forties.