My girlfriend has had strap-on sex with her last three boyfriends, two straight and one bi (me). I've never seen anal as a gay thing. It's a pleasure thing, whether you're straight, gay, or bi, top or bottom. With my gf it's been part of some really hot sessions. But from our own experiences and those of friends, the surest route to pegging comes when a dude is already fucking his gf in the ass and she's getting off on it as much as he does. If she accepts anal as normal, she'll already been interested in playing with her guy's ass. (When Cindy blows me, I always shoot a huge load because near the end she slides a couple of lubed fingers into my ass). So it seems natural for her to eventually want a cock hanging between her legs so she can fuck him the way he fucks her.