Ok, before I answer, I just want to say I'm not a homophobe. I'm actually a heterosexual male. So there are a lot of things about homosexuality that I don't understand. But I guess they decide who is 'on top' the same way you and your female partners decide and the same way my wife and I decide. (We wrestle for it. Isn't that how you decide? Sometimes we draw straws. But every now and then, when the wrestling ends in a draw, we simply do it side by side, with neither of us on top.)
As for your other question about experiencing pleasure, I'm guessing it must be pleasurable, since most people are not going to keep doing something that is not pleasurable for them.
I apologize for my crude responses.