Have you ever had a friend steak your boyfriend, girlfriend, spouse, or loved one?  If so, what happened and what did you do?

Had a brother sleep with one of my ex's... she and I had been together for several years but it had been over for more than a year when they "hooked up" and all I thought was "good for him!"  She was both hot and a tiger in the sack :-) and I had no more feelings for her.
I've said this in other threads though, if anyone I knew betrayed me by even hitting on my gf like that, I'd be done with them forever... never looking back and never thinking twice about them.