Firstly I think rape is a poor reproductive strategy. Consider - human beings are the most dangerous animals on the planet. Any woman who is in a position to raise a child is embedded in a social group of these animals. There is no way for a rapist to tell whether a woman is fertile or not. So, for a one-in 28 chance of impregnation, one takes on an organised social group of the most dangerous animals in the world. And while they have no restraint in the force they can use on you, you can't kill the woman or even injure her badly, or your reproductive strategy is fucked. And if you hit the jackpot, she survives, pregnant, with your child? No human society has ever been found where the females do not practice abortion.
OK, secondly, I don't think human sexuality is a reproductive function. It's like a leg, which may be descended from a fin, and may be used to swim, but isn't about swimming any more. Sex is about connection, and self assertion (I suppose at its best reconciling these two conflicting impulses).
I think rape starts at the outward edge of this, as a type of damaged self-assertion, and washes across all types of sexual expression - towards children, towards men, towards old ladies. Then sometimes it is directed towards fertile women, and then it might, very rarely, become a reproductive act. But that's not where it begins.