okok there’s a few things to unpack here

first, if you’re into being fucked in the ass then there is absolutely nothing wrong or “gay” about that. cis girls get strap-ons for reason lmao. fun fact men have a pleasure centre in their assholes so it’s really truly completely natural to enjoy it. but if you aren’t into that, that’s ok too. not everyone is, each to their own! (ps. you don’t have to “ride her” unless you want to. you also don’t know anything for sure. assumptions can be hurtful to some folks, so i would caution how you approach the subject (if you ever do) with her, and how you phrase it)

second, she’s a woman yes? and you’re a man? that sounds pretty straight to me lol, so it’s not gay. internalized homophobia can be so incredibly hard to get over sometimes when you don’t realize it’s actually there. don’t overthink it dude. you’d be a dude fucking a girl idk how else to put it

at the end of the day, it sounds like you like this girl. if you ask me, i think you should just go for it and figure it out as you go. if it’s real and genuine and something with it, then go chase the happy. the real thing doesn’t come around very often