I don't think that saying what one believes is necessarily being politically correct, nor is it a flaw. Yes maritial cheating is primarily held to be wrong in this country. That someone believes it themselves is not necessarily PC but simply their view.
Most people have poor reactions to being lied to. They carry that into others lying to someone. Failure to be honest about an outside relationship is lying by omission. I think the part I find troubling is that some seem to think its ok to pick and choose who they lie to. If today one chooses to lie to their spouse what keeps them from choosing to lie to someone else tomorrow? When does Master's name or submissives name move to the top of the "I'll lie to them today" list?
Its like the old joke about would you sleep with me for a million dollars? She says yes, he says how about for $10 and her response is what do you think I am? He says thats already been determined now we're haggling about price.
Same thing-its already been determined that the person will lie in a committed relationship. Now anyone else coming into the situation has to ask "if them, why not me"?
The fact is YOU are the one being PC. Do you have any idea what being politically correct means? You live in the USA... your fundamental politics dictates that marital affairs are cheating. So saying that a cheater is always a cheater IS being politically correct. It is an insidious flaw. Deal with it.