I'm not sure if I can believe him

I recently found my boyfriend going through my phone which is fine because I told him i have nothing to hide. I got to thinking it was odd so I checked his. I found links to dating websites and fling.com etc. along with other women sites and porn. I can get past the porn but why would a man who is committed go on a dating website? He says he doesn't know he must have been curious and that it was long ago. Initially he lied and said he was curious, then his friends told him to look, then it was something he saw on TV. He gets angry if I ask him questions and won't talk. Keeps telling me he loves me and only wants to be with me and it was just curiosity and he never registered or was looking for someone else. What should I think and how do I decide what to do???