What should I do (man issue)?

I did something silly. I have liked this guy from a distance for a while (nothing major, just always thought he was cute). Anyway, I was out at the weekend and got let down at the absolute last minute by a friend (I was already in the pub waiting). I decided to stay and have a couple of drinks because I have just finished exams and wanted to let my hair down. He started talking to me, we had a lot of fun, I got VERY drunk and one thing led to another. It was loads of fun and I'm glad it happened from an entertainment point of view. I thought I'd look him up on Facebook (having never done that before) the next day and discovered he has a freaking girlfriend. I was very annoyed (I'm not a fan of cheating and wouldn't have done anything with him if I'd known in advance) - he had plenty of opportunity to tell me. He showed no guilt or worry and I've left him with a pretty decent hickey that I'm wondering how he's intending to explain. Anyhoo...I sent him a message - nothing too angry - just saying that much as I had fun it's a shame to see that he has a girlfriend and I wished he'd given me the opportunity of having all the facts. It was a friendly enough message save for just making that point. I bumped into him today and he acted completely friendly and casual and didn't mention the message (he may well not have read it yet of course). What do I do? I really just want to know why he did it and why he doesn't seem to feel at all concerned about it? Of course, the answer is probably that he is a serial womaniser with no conscience but I'd be disappointed if that were the case. If he doesn't reply to my message can I go and see him in a week or so to get things off my chest or is that just going to make me look like a crazy person? Argh.