I've never written out slash (when it comes to males), although I've had characters swing both ways. I have no problem with the romance side of things, but I've been hesitant to ever put a character down that road because I don't think I'd be comfortable with writing out the sex scenes, and sadly that seems to be what many slash writers I've come across are in it for. A big generalization, I realize that, and the same could be said for het writers of course. My issue with the sex scenes isn't a question of morals or thinking it's wrong or anything like that, I think it's just the idea of anal sex regardless of the genders of those involved that makes me uncomfortable. So describing it makes my stomach turn. Lmao, too much information, I realize. But you asked, talkmod!
As for why it is popular? I really couldn't say, that is something I've never been able to understand. Perhaps a step outside the norm?