I would say, here is a reason why you should not do that: simply, men and women's private parts compliment each others anatomically and physiologically, whereas two men's don't. I remember someone once saying that when your heart (which I took to mean finding someone attractive) and your head (eg you know your body is designed for a female partner) don't agree then something's not right.
My biological instinct, though, when someone disagrees with me is to kick him around the field then take his food and his women... haven't we moved beyond the naturalistic fallacy?
Regarding which - sex is not purely about reproduction in humans. It's clear from the evolution of various sexual traits and tendencies that sexual activity has a social as well as reproductive aspect, and that social aspect doesn't solely involve 'biological compatibility'.
You might think that two male genitals are incompatible, but I suspect there are any number of gay men who find frotting pleasurable. Sex does not have to be penetrative to be pleasurable, but even if that's your thing there are perfectly possible variations available. You do know that sex isn't limited to the missionary position with the lights off solely for the purposes of procreation, right?
N.B. Just one my personal aggravations, but men's and women's genitals do not compliment each other, they complement each other.
O.