I've SERIOUSLY been wondering the same thing. Because my boy dog has nipples...and okay, I know it's retarded that I didn't know they had nipples, but anyway, it's dumb! He's a boy!
I'm sure men have them so that if the woman died during delivery the male could nurse them so the baby wouldn't die as well... with all the medical technology death durning labor isn't as big of worry, plus there are bottles and wet nurses these days. It has to be a back up plan from way back...