I'm completely with Skeezyks on this. If it is something as simple as looking at pictures on your computer, that in itself is nothing to be ashamed about. My boyfriend and I talk openly about what we like and dislike as far as other people, and each of us know that we have attraction to people other than each other. That doesn't mean that we don't love each other or that we're being unfaithful to each other. It simply means we're being honest about what we like in the opposite (and, in my case, also the same) sex.

I think a lot of the problem results from your girlfriend's previous relationship with her ex-husband. He committed infidelities towards her, and she probably suspects that, because you look at pictures of women other than her, you plan on committing the same infidelities that her previous partner did. I think it's obvious that she's very insecure about it. Maybe you two should go to couple's therapy and discuss this openly with a third, impartial party. I think it could be a great start in repairing the problems that exist in your relationship.