My boyfriend crossed the line..or did he?

I was on my laptop and I went to check my FB, and I saw that I had a message. I clicked on it and it was from some guy I didn't know saying how good my d!ck tastes, that is when I saw the other messages. This isn't my page! I kept reading and my boyfriend and this guy that had no FB page have been sexting each other for two months. They even exchanged phone numbers. Now I'm angry. We just got engaged... I want to punch someone, something! I know the sex life hasn't been the greatest. Couples go through this all of the time. I read the texts. It's all fantasy for him to get off. They didn't meet up that I know of... We're guys, we look at porn, we fantasize about a lot of sh!t ..but, I never actually acted upon them. What the f@ck should I do??!