For a lot of women, especially married ones, flirting with a married guy is "safe." They assume that it's not going anywhere, so they won't have to confront that whole "You came on to me, and now you're rejecting me" drama. Plus, if their husband finds out they're flirting with a married man, they're less likely to flip out, since they will also assume it's just harmless flirting. Whereas flirting with a single man means you're actually looking for something, and that can be dangerous.
That's the optimistic perspective.
There's also the downside that some of those women are married and seeking to cheat on their husbands. Cheating with other married people is also "safer" than cheating with single people, because "it doesn't mean anything, it's just sex." In other words, a married woman may feel she can cheat with a married man, and that he will not pressure her to leave her husband the way a single man might.
I am who I am. I don't need labels to define me. They're sticky, and I hate the glue they leave behind.