No one is a island! In such romanticism it's best said as It's ok to be the parmesan added to a married woman's hot, hot spaghetti, to just melt together makes something great! In other words making a better place in polyamory for those involved. It's ok for a married woman to have a lover. Keep in mind it's not only about sex more like he is there to give her calm and be a vent for her, she may not have otherwise, all around better things to make her better!
I had a married love that would come over to see me when her and husband had a fight. I listened to her, just snuggled and in time calmed her down getting her out of my place a stable woman. She liked it when I genteelly put my hand on her back and pushed her out my door to go back to her husband. Then I would look at her walking down the walkway inflated in a better place. And is the point of Polyamory!