Both, I guess. Plus the fact that I got yelled at for getting upset about it when I found out about it. That's probably the main reason, actually (now that I'm talking about it). Never felt like he was sorry.
I still stand by what I said though. I don't think you can have love without trust. If you tell someone you love them and that you want no one else, then you need to stick by it. I would break up with someone before I ever had sex with someone else.