I've had 3 friends in this sort of situation.
My female BF, was the OW. "He" was her tutor at college (she was a mature student). The "affair" went on for 8 years, he left his wife for her several times and went back, eventually he left his wife for good and they have been blissfully married for 9 years. He walked away when his children were old enough for him to explain to them (late teens) and after giving his 1st wife everything - house, finances, the lot.
I would be genuinely shocked if he was cheating on my BF. She has an excellent relationship with his 2 adult sons and their OH's, but has stated it is a bit awkward at his family gatherings if his 1st wife is present. They are close friends of DH and I and are open about the circumstances in which they met, although they don't broadcast it widely.
My other "friend" is the cheating "D"H. He has had 4 affairs to my knowledge (not with me, I hasten to add, he was someone I knew at work) - all long term-ish, 3+ years. At the moment, he's trying to make a go of his sham of a marriage, because he has 3 teenage children and he doesn't want to be a "Saturday dad". His wife knows about 2 affairs.
He has no intention of ever leaving for whatever OW he has at the time of any affair, and is actually a right bit of a cunt. He got his DW pregnant in the middle of a 5 year affair with an OW, and told the OW it was "an accident"... OW went off the rails and had to move jobs because she thought he'd leave his wife for her so they could start a family. He claims he's "unhappy" but stays for the children.
He actually stays because he would be screwed financially if he and his wife divorced.
The 3rd friend, has just divorced at 50. Her ex-H screwed around in her marriage, had numerous affairs and really destroyed her. She is now having an affair with a man 15 years her junior, who is married with 2 small children. She claims she's not having an affair because she's not married and he's the one cheating. I've stopped taking her phone calls because I am fed up of her analysing how she is in the right to shag the married man. She's churning out all the cliches about how he's not understood by his wife, etc, etc. I'm not interested.
I don't judge any of them or their relationships, because it's not my place to, and my experience of different friends suggest that it's not black and white as it seems. BUT DH and I have discussed affairs with regards to our relationship and are both of the opinion that for us, an affair by either of us is an absolute deal breaker and that would be the end of our relationship.