Hi unplugme
I was a Mistress for 3 years, his wife wasn't interested in sexual intimacy. We worked together and it was apparent from day one we had sexual chemistry and we gave into it. Other than friendship we didn't have feelings for each other to start with and I would actually give him grief for not supporting her at times of stress.
But it did get complicated with emotions, he began telling me that he was married to the wrong woman

I had no intention of being a home wrecker but it was heading that way so I stepped away so he could clearly think. We didn't restart our relationship despite his coming clean to his wife. They separated and it turned out she had been having an affair for almost as long as ours had been.
The point of my honesty is that their marriage was in trouble b4 me but my involvement prolonged the unhappy marriage. I would seek advice and therapy b4 going elsewhere for intimacy, it may prolong a situation that is unhealthy for all parties.
Good luck
PS. . . .
To anyone who feels the need to judge or berate me for my previous life choices I will kindly point out that people in glass houses should not throw stones.