Because they are "VERIFIED" as worthy males since some woman took them off the market, they are seen as a high SMV individual. Kinda like how some guys here want to steal a girl away from her boyfriend, women get competitive too.
Women will often be a little more flirty with a married guy, because there is less likelihood of him calling her hand, unless he is REALLY interested. I think a lot of women like having unavailable orbiters to feed their egos.
In the final years of my marriage I noticed more affectionate overtures from women. Taken women more than single. Things like "Love you " written in the dust on my back car window, slapping my butt, finding excuses to hug me, posing with me in pictures in provocative poses. But I welcomed it and I treated all these women like they were Little Sisters (in the Fraternity sense rather than related). Really I craved the attention and the affection since I wasn't getting it at home.
How did I screen? Mainly from how removed they were from my social circle with my wife and work. One woman (who I never had anything with by the way, but she sent plenty of signals) said "i don't believe there is really a Mrs. Fuck This. You are just making her up". If that isn't a signal that she was available, I don't know what was.
Mainly it was when the pain point for ending the relationship point became roughly equivalent to staying married. And the perceived benefit of being involved with the woman would warrant the pain.
I remember thinking at one point "This woman could really fuck up my life." Then the next thought was "Well don't think it could get much worse...."