Maybe. It would depend on how it went down.
I met a girl while I was in a relationship once. I met her while on vacation and it turned out we were from the same city. We hit it off and things escalated pretty quickly. I ended up cheating on my GF with her but I never divulged that I was attached.
As soon as I got home I ended it my GF. At first I never really gave a reason, I just said that I wanted to see other people. She asked if I met someone else. I said yes and that was it.
While I wouldn't exactly say that I acted honourably I don't really feel any shame about what I did. Besides everything ended up working out in the end. I'm still friends with the X and the relationship with the new girl went well. I even eventually revealed to her that I wasn't exactly single when we met. Surprisingly, she was cool with it, although she was a little miffed that I didn't tell her straight up. She actually even found the idea of being "the other women" kind of hot.
I suppose if it happened something like that I could see past it. I don't think I would if she was deliberately hiding it from her SO while staying with him for any period. That shit would give me trust issues for sure.