I'm falling for my best friend

So, there's a guy I've known for about two years now. He recently became my best friend. He has this girlfriend in another state. She cheated on him a few times in a previous relationship of theirs but he forgave her. Our relationship has been nothing more than innocent. Until his parents went out of town and we got a bit too tipsy. We were laughing and dancing to music that was way too loud. And then a certain song came on and I started dancing on him. When the song was over he picked me up and carried me up the stairs to his bedroom and I shit you not it was the best sex of my life. Afterwards we went outside for a ciggarette. And I remembered he had a girlfriend. I reminded him and he pretty much had an all out melt down. He felt like such shit. And So did I. I told him he needed to figure out what he wanted and whatever it was i supported it. Even if it wasn't me OR her. The other night, he told me I was all he ever wanted, but he hasn't left her. He says he doesn't want to destroy her. From why I understand her happiness is based solely on him. I just hate being the other girl you know. Now normally hearing this story is be like yeah that guys a piece of shit playing you both. But he's not. He's the best person I've ever met. He's my best friend, I get him, he gets me. I can see this whole thing really messing with him. He's such a nice kid and he feels SOOO bad about it. Sorry about the rant I guess I'm just hoping your opinions are what I want to hear.