Many young people would have some idea, rural teenagers in partciular would observe animals mating, giving birth, for example. Also, the majority of people lived in small homes, and sharing rooms was the norn. Children might easily overhear their parents lovemaking, observe it even.
This would be less likely with the more cossetted upper and middle class children, but even they could probably get some idea. For instance, in her autobiography The joy of the Snow, the novelist Elizabeth goudge wrote that her own mother (born in the 1870s) worked it out from passages in the bible, and explained it to her sisters. Mothers of daughters who were going out to work, as domestic servants etc, would warn their daughters against the dangers of premarital sex.
Again, the more protected and chaperoned upper and middle class girls would be much less likely to find themselves in a situation where they needed such warnings, but a mother might tell her daughter the facts of life before she was married, though apparently to some young brides it came as a complete surprise. It was different for boys of course, who might have an opporotunity to visit prostitutes. It was also not uncommon, at least among the upper classes, for a young man to gain experience through an affair with an older married woman (wives had more liberty than single girls, and provided they were discreet, it was often accepted).
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.