Just because people talk about sex, does not mean that they necessarily want to have sex with the person they are conversing with. Unless he suggested that he wants to do things with you, then try to proceed as normally. Don't think too hard about it and pretend that it never happened until he brings it up. If he tried to be suggestive, then I'd stay away from this guy. He has a girlfriend yet he wants to do things with you? No decent person would deliberately try to do that, hence he is not worth your time and you should try not to talk to him.
I like this guy but he has a girlfriend. But yesterday we were talking about sex and stuff like that for 12 hours straight! What do I do?
So he was talking about sex? That doesn't mean anything, that just sounds like any other young lad who love to talk about sex! :) One thing I've learnt this week is that guys show a lotta interest in a girl but 80% of the time it means nothing.
12 hours though is a long time...but still I wouldn't assume he likes you because trust me this week I've been in the same situation (only that we never talked about sex) but since he has a girlfriend it's a difficult one anyway. Are you expecting him to leave his girlfriend for you? I wouldn't count on it...just because they act interested doesn't mean they are. Find a guy that is available, or better still talk to this guy and if he gets closer then wait for him to leave his gf or whatever but do not wait long and do not count on it because you'll be wasting your time. After all you don't want to fall for a guy that wont be chasing you :(
Sorry If I'm being negative, just trying to be honest. :) Good Luck.