A couple of times in my 20s. One lasted for a year. It was just sex, not a love affair. He very much loved his wife and talked highly about her. They were TTC, so he thought of sex with her as duty sex, I guess, but not in a bad way. He had other women too. I wasn't the only one. So, if he wanted a blow job he was going to have to hit up one of the other women because I sure as hell wasn't going to do that in this situation.
After a year he had a job opening at his office and asked me if I wanted it on the condition we stop the sex as he drew a line at sex with colleagues. It was my choice, continue the sex or quit the sex and go to work for him. The sex was great, but I thought it had run its course, so I went to work for him. They are still married from what I hear and had one child. This was 20+ years ago. I worked for him about a year. No one ever knew our past.
The 2nd one was a surgeon who was in town for a convention. We met in a bar on his 1st night in town. We spent the next 4 days naked in bed in his hotel - other than the speech he came to town to give. No sex. Just talking. He loved his wife, too. She was his former nurse. But they had a severely disabled child and they were both very stressed out. He didn't think he was cheating since we didn't have sex. He just wanted to talk. I liked him. I thought he was a nice guy. He started sending me love letters afterward, so I cut him off because there was no point in that. That disabled child died. He eventually divorced and remarried (this I know from Google stalking -we have not been in touch - I doubt he even remembers my name).
Since then, married men hit on me all the time, but I am not interested. It's just so cliche and I hate this idea of these men getting their cake and eating it too. I've never been married and I am in my 50s. I don't think my experiences are typical. I'm pretty cold. If either of these wives would have called me I would have said "Stay with him, he loves you, what we were doing was meaningless."
The 1st guy will always have a harem going, no question about it. The 2nd guy seemed a little out of his comfort zone. The 2nd man's cousin called me and asked if I would go out with him if I wasn't going to "see" his cousin the married surgeon. The cousin was single, but I declined. Seemed potentially messy.