I imagine that they like to peg for similar reasons to why men enjoy dominating woman. Personally, I enjoy the fantasy of the complete relinquishing of control to a woman and the act of being dominated by them. But it's always a woman in those fantasies. The thought of being fucked seven ways from Sunday by a smoking hot woman with a prosthetic toy (or their fist for that matter) is something that never fails to get me worked up. But, at the age of 43, the idea of sex with a man or a trans woman has never even illicited the merest of twitches. I'm not so hung up on labels and insecure in my sexuality as to think that, should the idea have any appeal, I'd be adverse to trying it out. But M/M or M/TS porn just does absolutely nothing for me.