I would say that you don't quite know what your line is until you get there. Before I knew about H's A, I would have said that him having an A would end our marriage right there. That was my line. Then I found out about the A and my line moved. I was willing to forgive and move on as long as he ended the A. The unfortunate thing is that he never ended the A. He continued/s his involvment w/ her and he lies about it. Because of that, my feelings and respect for him are pretty much nothing.
Why am I still hanging on? I've been asking myself that a lot lately. For me, it still comes down to the fact that I wouldn't be able to live with myself or look at my kids if I were the one to put the final nail in the coffin of this M. My H is very likely going to end our M, but it will be by his doing, not mine.