You got a significant other, see? You’ve also got this great best friend of many years. One day you discover the significant other, whom you love, has been laying pipe with best friend.
What do you do?
When it happened to me, years ago, I somewhat rearranged the skeletal structure of my best friend, who had instantly become First Enemy, ignoring excuses. Significant other was promptly tossed out on her well rounded ass, shortly followed by everything she had kept at my place, having, in my view, abruptly become ‘hoe #1.’
There were no excuses good enough for me to consider taking either of them back.
I heard this discussion on a program, and guys in general seemed to think it was fine if they cheated, but not so fine if their girls did. Girls disagreed, but felt that there was no real harm in cheating with the best friend if there was a reason.
Some said best friend should tell guy that SO was actually a slut for sleeping with him, even if SO started it. Some said best friend was a cad for going to bed with the girl no matter what she did. Some girls said at least it was the best friend instead of a stranger from a bar. Some guys said when sex calls, it’s basically a Man’s Duty to respond, and then let the boyfriend know that girl is a cheater, because as a best friend, it was an obligation to point this out to the one cheated on.
Some of both sexes said shut up and keep screwing.
Some would forgive and forget. Some would not.
My opinion is that a best friend screwing one’s SO is worse than it being some guy from a bar, for the best friend should be loyal enough not to do that under any circumstances. (Which was why his beating was a bit more thorough than one administered to a stranger in a bar fight.) I’ve turned down passes from friends SO’s before, admittedly at times with difficulty, but loyalty is loyalty.