How do I get myself out of this one?!

Ok everyone! So there’s this guy that I’ve met through work and way back (few months ago) he asked me out a few times. I found out he had a girlfriend and straight away told him that i was not interested. Some time has passed and every time I see him, he now tries (and often succeeds) to touch me or kiss me.

I feel bad because he has a girlfriend (who he keeps saying he’s not sure about), but I in a way kind of like it, there’s a thrill about it.

Am I a bad person? How do I get out of this situation? I’ve told him I can’t be his side girl, but I don’t want to cut him off and be a bitch. What do I do?