Well first things first, I'm heterosexual and as such I'm not attracted to the same sex at all so having sex with a man is out of the question. Well if it's not obvious enough, then let me say that I prefer and only have sex with the opposite sex and that's because I'm not gay and neither am I bisexual.
Well I've never even remotely had anything close to a sexual relationship with a man so I can't actually draw a basis for comparison between having sex with a man and having sex with a woman, but I'll say this, guys are usually stronger than women, and I don't mean strong as in strength, I mean their bodies are literally harder than women's bodies because alot of guys work out and guys naturally have bigger muscles than women. Personally I like soft people and women are usually softer than men so I'm sticking with women.
Moreover, I like having things to hold on to and I'm not interested in grabbing a muscular guys pumped up chest, it's breasts for me all the way. Oh and let's not forget that the ideal figure for a guy is a rock hard chest, chiseled abs, toned pecs and an all-round fit body, while for women it's a big butt and moderate to large boobs and a flat tummy. I'd pick the softer of the two to have sex with and that's the women.
Finally let's not forget that only one sex has a natural access point for the male genitalia and thus any other access point wasn't naturally made for sex. So once again I'll stick with the sex that has the natural access point and that's the females.
I hope this helps.