Idk what to do..

Caught my fiancé jacking off to pictures of his best friends girlfriend..

My fiancé and this girl we will call Lisa has been in a relationship a few years ago. Lisa is the first girl he really had a thing with. Lisa is one to show off her assets. She was blessed with a nice rack and I feel insecure around her because I was not.. My fiancé talks about when they dated often and “funny stories” about times he would grope her. I am very open with things that I am not okay with, like him talking about her like that to me. He spends a lot of time over at his friends house while she is there and gets upset when I show up. I’ve been concerned about her for awhile and he just pushes it off and says he doesn’t think of her that way anymore until I caught him jacking off and when I glanced at his phone it was revealing pictures of her from her Instagram page.

What should I say to him? I don’t believe he is cheating on my because she is respectful of the relationship I have with him or at least I would think..? I feel sick to my stomach I couldn’t sleep next to him.