OP, you really need to stop trying to rationalize and justify this, it's wrong. What both of you guys are doing is wrong. You're almost as bad as he is, because you ARE a willing part of this equation, you're pretty much just hanging around until he decides to go all the way. And he is a jerk, there is no way around that.

Let's say for the the sake of pretending that he is honestly just this poor, confused man, who found someone he has a connection with yet is tied down to another woman. Guess what? He's still an ass. He could be intentionally playing mind games and pushing your boundaries to see how far he could go (or wait to see how long until you literally throw yourself on him), or he could be legit (i doubt it) a confused soul, but being one or the other doesn't change that fact that he is slime. Don't think you're special, you're not, you're just new and available.

Your conflict doesn't so much seem to be in the fact that he has a girlfriend (because it seems you obviously don't care) but the fact that he hasn't completely taken advantage of you physically, therefore tricking you into thinking he's genuinely a good guy. It's called pushing boundaries, playing mind games, and loving the chase. It's really not that complicated.

All that being said, you should really change your mindset, not all men are dogs and not all men cheat, but you should really see yourself as that special in order for that to happen. Dont let yourself be the other woman, don't let yourself be second choice, gtfo of this mans life and get yourself a single man.