Personally, I just don't like contributing to someone else's problem. I sleep better and it's not like there aren't enough women on the planet. Like I really need to deal with some skank who would cheat in the first place.
Standards start with yourself. For me, that's no married women and no girls with boyfriends if i know the boyfriend. If I dont it goes on a case by case basis. I will entertain engaging in a particular kink I'm into with married and attached women so long as there is no sex involved, but that's only happened once with a married one and I doubt I will again. The sneaking around is a royal pain and the one I met tried to **** me anyway. Really don't need to deal with it. Come to think of it, there was a seprated one, also, that I didn't know was still married until we met...tried to **** me also. Just really dont need to deal with it. There are enough divorced women who act like they got out of cages if you need that level of desperation.
I have friends who stray in relationships and who mess around with women who are taken. I don't judge. But I'm not taking a bullet for them if it ever comes down to that.
Do your thing. If you think what you are doing is fine, have at it. You don't need those of us who don't to make you feel better about it, do you?
Although I would submit that the "doing the guy a favor" thing is just a wee bit of a rationalization. Right up there with "if it was working I wouldn't have cheated" or "it just happpened" or all manner of chick logic designed to absolve oneself of guilt for ones own behavior. If you sleep okay as a cheater, just own it imo.