I think the people who think that they would not be pissed at the other guy are being unrealistic. I pretty much agree with opiate's view. It would be nice if the average man respected the institution of marriage enough to not bang married women, especially given the vast amounts of available snapper out there, but the world is a shitty place. This comes back to the morality discussion we had a year ago, regarding would people nail a married women if they knew they could get away with it or would they cheat if they knew they could get a way with it. It comes back to what kind of person you are and the sort of asshole who knowingly plows married women deserves some ration of shit for being a weak scumbag of a man. While I would not personally engage in such behavior, I know and accept that there are men (and women) who cannot say know to their dick and do one time stupid shit, so I doubt I would go nuclear on the guy, unless he had a history as a home wrecker.
Actual rage is reserved for the kind of pond scum who goes out and forges relationships with married women. These men are parasites and worthless human beings, usually careening from one dumpster fire of a relationship to another. No man should enter a relationship with a woman who is married, because if she is cheating on the current man she will cheat on your dumb ass too. Or worse, she goes back to the guy and dishes on your affair when they have a shitty argument to score points and now you have some dude looking to come after you over your shitty behavior. If the woman is unhappy and in the process of ending the current relationship, fine, but only if you are 100% certain that the soon to be ex knows and accepts that the impending divorce. And it goes without saying you go scorched earth on the disloyal cunt who broke vows in this situation, even with kids. Going full pussy in this situation is going to make your kids think you are a dickless pushover.
If the other guy has no clue she is married, then no harm no foul on him. Odds are two Walmart work buddies are going to know each other's marital status, however.