How would you react?

So a few days ago, my boyfriend confessed to me that he had been texting back and forth with a girl from tinder that he's never met. He tells me he's "sexually frustrated" and that he flirted with her but didn't say anything blatantly sexual. I don't react for a couple hours, but when I do, I'm pissed (guys have cheated on me before.) I talk about it with him and he cries and tells me that he knows it was stupid and he doesn't know why he did it and that he knows I'm the one he's supposed to be with, and he doesn't want to mess up what we have. Seeing him cry took out any fight I had in me, but I still think about the girl. Like I can't get her out of my head. The argument is over, obviously, and I don't want to bring it up again, but it's just been bugging me to death. 
What do y'all think?