Humans on a basic level, to keep their own society and environment flourishing, need these three things, but humans are so much more complicated than animals. WE build roads, WE fly planes, WE drive cars, WE learn the intricacies of the human body AND the animal body, the nuances between subatomic particles, WE manufacture devices that allow us to communicate thousands of miles across the world. Humans are advanced and with advancements comes complexity. In terms of sex, the reason men may want vaginal despite loving anal isn't simply that he wants to procreate. I can tell you that I certainly do NOT want kids at this moment, but when I have sex, I'd like to know that if I choose to nut inside a woman, that there is a 100% chance she won't get pregnant (i.e. I fuck for FUN, not to have offspring). I've heard reasons for why men may not even want anal sex; it's gay, it's dirty, she has a pussy--why fuck her ass? As for men who still want pussy after claiming to be all about ass, I don't know. I usually hear things like, "There's nothing like a pussy".
I can only speak for men on this, but we aren't as easy to explain in this regard. A bunch of men here are anal only for a reason, and having children wasn't one of them. I wasn't always AO, but when I was interested in sex, having kids was not on my to-do list. What's funny is that some women think a man wanting to fuck her in the ass is gay too. For me, it's that you have a hole sitting between two halves of a body part that I LOVE so much. Why not give it a try? You wanna talk about how tight your pussy is? Your ass is tighter, I guarantee it. So what if you need prep? Nothing is worth having if you don't work for it.