Is it normal for my boyfriend to be on craigslist?

This past year he's moved from porno to his addiction of craigslist. I found him emailing some girl in january about how much she cost. I was upset and asked him about it. He denied it, but I kept pushing it until he said he was just curious... well I also found out this has been going on for 2 years now, we have been together 4 years. And are thinking about marriage but that is out the window for me now. Well lately I've had this ugly feeling like something was going on and so I went through his email this morning and found that he was replying to a guys ad for sex on craigslist. I am so sickened, and I literally threw up. I confronted him and he once again said he was just curious and that he didn't do anything. That he won't since he's only been with me, I devirginized him back in the high school era. Is it because he's only been with me? Is he just curious? I don't know what to do I told him what is he thinking those people probably have a disease. This is the only thing that has been destroying us. He tells me he loves me and usually spoils me I just don't know whats going on now. I have not spoken to him I am laying sick to my stomach in bed just thinking about it. I don't know if i can look at him the same ever again...