He cheated. What do I do?

I've been with my boyfriend for almost 4 years now. We got engaged a few months ago and we have been planning our wedding and whatnot. Yesterday I found that a girl sent him a nude which obviously broke my heart. I honestly never thought he was the kind of person who would do that. Our relationship was so great that I would never have suspected it. Now I have to make the decision of staying with him or leaving him. I really need advice

Vote below to see results!