He feels guilty. That’s why he’s pretending like your little romantic evening never happened. Because he knows that if his girlfriend found out about the kissing, there would more than just “problems.”
Hanging out with someone you have feelings for is the WORST thing you can do after a fight. He should have gone out with his boys and blown off some steam, or hung out with couple friends who can give advice. If he hangs out with a female friend, her boyfriend had sure better be around. Reading your question, “crying in the closet and falling asleep” was not where I expected things to go. I’ll give him credit for not sleeping with you, and for feeling remorseful, but he still never should have come over. Chances are, he knew you had feelings for him before he came over. He wanted to be around someone who would make him feel better about himself. You didn’t have sex, but he still cheated by kissing you and sleeping in the same bed. Putting aside the kiss, his first mistake was coming over to your place instead of meeting you in a public place. Or, better yet, hanging out with someone he wouldn’t potentially drunkenly make out with.
It’s pretty crummy that he’s pretending like nothing happened. Not to mention disrespectful to you and the girlfriend. He thinks you’ll be fine with forgetting his little indiscretion. Maybe you don’t want to be part of his drama. Definitely clear the air. Say that it was a mistake for you guys to confess your feelings for each other and also kiss while he was having issues with his girlfriend. Also, let him know that you wish he’d man up, confess to his girlfriend, and apologize to you for playing on your feelings for him. If he plans to stay in the relationship, you guys need to consider a) whether he should tell the girlfriend or not and b) whether you can still remain friends. Because there will be other fights. Perhaps worse than the one that sent him on a kissing and crying bender.
Ask him if he’s really happy in the relationship. Sounds like this was a one time thing, but you never know. It’s something he should address. Whatever happens, your friendship has now changed. You probably should take time away from each other. It’s not fair for you to be around his girlfriend after what happened. (Chances are he’s going to keep you two apart anyway.)
Good luck. And if a friend comes over looking for relationship advice in the future, hide the booze.