Well let's be clear, it's not really a gay scene. For it to be a gay scene, both the characters would have to be gay. But in this case, the MC isn't gay. But the guy he's fucking is. So it's not like, full-on gay.
It's been pretty firmly established that you're not gay if you're the one on top. Unless of course you're a power bottom. Then it's ok to be on the bottom because you're the one generating all the actual thrust.
What can I say, there are a lot of rules and I didn't make them. It's like, what is and isn't NTR. A lot of fine lines.