Should I be mad?

So me and my BabyDaddy/Fiance have a mutual trust for eachother and I know he has a female best friend he's talked to since forever and I've respected that as long as she respected us. (He was a virgin when we met) his brother mentioned something that made me start feeling a bit off about this girl and I scrolled through his messages with her. Turns out they were sexting.. it was before we met and she has a man and a baby now he says there are no feelings between them of that nature and that was a long time ago that they're just friends. I know he hasn't talked to this girl sexually in a while but they still talk. Is it wrong that I feel pissed? What would y'all do in this situation.