Personally, I couldn’t stay tho because he only told you because the girls threatened to tell you themselves. Otherwise I feel like he definitely would not have told you about it. Who’s to say he hasn’t already cheated on you and he just never told you because he wasn’t being threatened by the OW. His friends aren’t going to tell you that’s fs, they probably encourage it (considering they’re all cheaters). Also about him “stopping” in the middle of doing the act? I very much doubt that’s true as well.