I'll add my voice to the people defending how the OP could have not known about the girlfriend. When I was in college I liked a guy who was part of my circle of friends. Between sharing classes and getting dinner together as a group with other friends, I saw and talked to him nearly every day, and yet had no clue he was dating a girl from his home town until he told me after I asked him out. Of course, I backed off but we remained friends. A year later, they broke up and once again I didn't hear a peep about it. He just out of the blue asked me out and I had to flat out ask him "What about you and (ex's name)?" before I found out they were over. So yeah, it's quite possible to be well acquainted with someone for a long time and not know about their relationship status, especially if that someone is unscrupulous and actively tries to hide it. Seriously sucks, though, OP. Now you know not to waste any more time on him, though! You don't want to be with a cheater like him.

Methinks OP must have known something if she's "liked him for ages". I smell a YDI.

been there hun.

So you didn't think to ask "oh hey are you seeing anyone?" before humping him?

Don't blame yourself OP he's the one who never said anything. He's scum.

Sorry to sound like an ass and he's totally in the wrong to but kinda a **** thing to do

Honestly OP could have assumed he was single if he acted single. She could have flirted with him and when he responded positively saw it as a green light. Not saying I approve of this method because of the lack of efficiency but could happen.

If the guy acted single then he is an ass. If OP still sleeps with him after knowing he has a gf, then they deserve each other.

Yay, you're an unintentional ****. Well you and his girlfriend can whack him into the ground when she gets the news.

He’s talking in his sleep. Probably doesn’t mean anything.