What you do is nod politely and back away slowly, because red flags do not come brighter, bigger or more insistent than a dude with a girlfriend telling you he loves you.
If he sincerely does love you, or at least likes you better, and he's in a bad relationship, he needs to do the right thing and break up with his girlfriend before he goes after you. If he's just looking to get in your pants, well, he'll find someone else. But either way, insist on his being unattached before you have anything to do with him. It's best that way for both of you.
Follow your gut reaction, which is obviously what led you here to ask that question.
Who's to say he won't do the same thing to you if you are ever his girlfriend? Be wary.
If he loved you so much, he'd be with you, not his current girlfriend. End of story.
Hmmm, but would you really want to be with a guy who changes out women like underwear? I think LA there has a very good point and the OP should move on.
Well, I think there are SOME cases where two people who are perfect for each other meet at the wrong time (ie when one or both are already in a relationship).
I agree though, chances are that's not the case, and he's just an indecisive a-hole (especially since this guy is still with his girlfriend while he's telling OP that he loves her).
Anyways, is it just me or are there way too may questions on here along the lines of 'I met this perfectttt guy. Only problem? he has a girlfriend/wife/family'. Come on, people. You know the answer.
If he'll cheat with you, he'll cheat on you.