Here's a nice metaphor for the theory of population genetics by Richard Lewontin (1974. The Genetic Basis of Evolutionary Change, Columbia University Press, New York, p. 189):
“For many years population genetics was an immensely rich and powerful theory with virtually no suitable facts on which to operate. It was like a complex and exquisite machine, designed to process a raw material that no one had succeeded in mining. Occasionally some unusually clever or lucky prospector would come upon a natural outcrop of high-grade ore, and part of the machinery would be started up to prove to its backers that it really would work. But for the most part the machine was left to the engineers, forever tinkering, forever making improvements, in anticipation of the day when it would be called upon to carry out full production. Quite suddenly the situation has changed. The mother-lode has been tapped and facts in profusion have been poured into the hoppers of this theory machine. And from the other end has issued–nothing. It is not that the machine does not work, for a great clashing of gears is clearly audible, if not deafening, but it somehow cannot transform into a finished product the great volume of raw material that has been provided.”
Pupils often ask questions that advanced learners would never think up. One such question during sexual education was, whether a fetus could be harmed through sex during pregnancy. The answer is no, of course, if the missionary position was avoided.
At that occasion I remembered the recently renewed interest, on the Internet, in the question why human males have no penis bone
(baculum) and associated that scientific question with the pupil's question.
Look at the bacula of rats voles and squirrels featured at the Loom for example. The mere shape of these bones makes copula during pregnancy seem highly risky for the fetuses. Unless some sperm/mate competition scenario was conjured up, I'd bet these critters never mate during pregnancy.
The bacula of other mammals are not as spiny, pointed or barbed. Here's one from a raccoon.
Raccoon baculum by Mordicai (GFDL, Creative Commons)
Nevertheless, if human males happened to have anything remotely similar, who knows, teachers around the world might conventionally warn pupils of sex during pregnancy instead.
"In conclusion, the impression one obtains of sexuality as a method of reproduction is that it represents protoplasts engaged in reproduction under peculiar difficulties that do not obtain in reproduction by spores or by vegetative multiplication, and that its significance lies in the fact that it makes organic evolution more rapid and far more varied." John Merle Coulter (1914, p. 137: The evolution of sex in plants. The University of Chicago Press)
"A simple explanation is one which invokes causes whose nature is immediately apparent to an untrained observer. Natural selection is a simple theory because it can be understood by everybody; to misunderstand it requires special training." (Graham Bell 1982, p. 81. The masterpiece of nature. London: Croom Helm)