A guy with a girlfriend likes me and I like him, what do I do?

[Manna]

I've known this guy since late August. We were flirty from the start. I then found out he has a girlfriend. One day whilst Facebook chatting, he gave me his phone number, and we began texting. Eventually he finds out I like him.

Late at night, on two occasions, we sexted. I feel bad about this now. He also began asking for topless photos, which I never sent. I eventually sent a text telling him that I will not send him photos while he still has a girlfriend, and in response told me he had been waiting for my reaction. Wtf, right?

Yesterday, I finally got the balls to message him that I would no longer be sexting him either, because I felt bad since he has a girlfriend, plus I didn't even know if he liked me back! So, tonight, he tells me he likes me back. Now I don't know what to do.

I don't want to break him and his girlfriend up. Seriously. I don't want to be a man stealer, or a home wrecker. But, if him and his girlfriend broke up and he came to me, I wouldn't know what to do. I know if he's doing this to his girlfriend now, he'd do it to me, but I still really like him. I feel like an idiot!

So, what should I do? Keep talking to him? Back off? Just wait and see what happens? I have no idea, and nobody I know has experience in this. Please help! Thanks.
No rude comments, please! Thank you!