My STBXAH met a 'friend' on-line that he started hanging out with. He swore she was just a friend and I, like a fool, trusted him completely. I didn't lose a minute's sleep over it (after all, he was an alcoholic with a phobia about washing more than once a week). Hah. I judged him by what I (or the man he used to be) would do in that situation rather than what the man he had turned into would do. Just before he left he told me he was 'in love' with her - all the while swearing nothing was actually going on. 2 months after moving into his own place, she moved in with him. I'm still trying to let go of the anger I feel towards him. I trusted him far too much - I will never trust anyone that completely again. And I think that's a good thing!
Infidelity, acted on or not, was/is my deal breaker. Only you can say how much you can tolerate.