Why invest yourself in someone who is not trustworthy? If the guy will fuck around on his wife, he will fuck around on you. Plain and simple.
You beat me to it, badger!Of course, this is assuming that a married man is being unfaithful. If he's in a polyamorous relationship, that's different. (And yes, I know that some people don't want to deal with polyamory - s'okay. Some folks are monogamous, some are polyamorous.)
But cheating is simply a bad idea.
I have done so many married men i get the group rate.