Why does my man go on dating websites?

We've been together for two years. This is the second time I've caught him... He repeats he's never.met or touched anyone else but me. But goes there out of curiosity or he doesn't know why he goes there. Yes we have bombbbbbb ahhh sex. We live together and our life is progressing pretty well. We have a home and jobs and I'm going to school. I just feel like I don't know where it went wrong? :( I am so disappointed and hurt that I'm shocked.