There are even more interesting implications of the "war": women are fertilized inside the body so that fathers can not be sure whose offspring a child is (because the fertilization can not be seen, other than with eggs, for example) - otherwise they might kill the offspring of other fathers (our ancestors would have, that is). Furthermore, women try to hide their fertility as much as possible, other than most other mammals. Iirc that is for the same reason: since men can not be sure when the woman was fertile, they can not be sure who made her pregnant. But if they don't know when a woman is pregnant, why even have sex? And that is why sex is fun. We take it for granted, but apparently it is only fun for very few species.

I recommend "Why sex is fun" by Jared Diamond for a fun read... I hope I have remembered correctly (it's been too long since I read it).