Am I a commitment-phobe?

A couple months ago I started talking to this guy. I let him know right from the begining that I didn't want to be tied down or commited to anyone for the time being. He said that was fine and then a week later asked me to be his girlfriend. I declined, he replied with an ultimatum "be my girlfriend or get out of my life." I was perfectly content with fooling around I don't need a relationship but he doesn't get it. I didn't say anything to him except I'll think about it and that night he caught me with his best friend. I don't know what my problem is. I feel bad but I also didn't do anything wrong! Two nights ago (3 months after the incident) he called me and asked if we could meet up to talk. We did and everything went fine but now he wants to be with me again and I really really like him but I don't know what to do.