I've been the OW.
In the first instance he wasn't married but in a LTR. I justified it to myself because I was madly in love with him. I was young and vulnerable and he made a move on me. I continued to justify it because I was the only one who got hurt, she never knew. He broke my heart, twice, and it fucked me up for a long time. I got what I deserved there.
He's now married to someone else.
Second time was different, he was separated, I was in a LTR. I can't justify this one. I don't really know what to say. I was confident my partner wouldn't find out and I guess for that reason I didn't feel as guilty/awful about him as I should have.
The guy I was seeing got back together with his wife, I found this out from someone else.
His reputation preceded him, he'd done it before and still was while seeing me. He apparently had an agreement with his wife that he could shag around due to a very high sex drive. Again, this info didn't come from him.
I never, in a million years, thought I would be this person. I thought I had a strong moral compass. I thought I was better than this. But apparently I'm not.