When I was 18, my girlfriend cheated on me with one of my really "good" friends. It always takes two to tango but my friend (who also had a girlfriend of his own) was the one who instigated it while I was away overseas. It felt like absolute shit and I didn't really trust women very much for a long time, because to add fuel to the fire she lied to me about the cheating. She just claimed that they made out, when in actual fact I found out that they had sex.
He tried to get one of my other friends to do it at first, who replied with "Fuck you, I don't want to screw my best friend over." So he decided to do it instead. Truth of the matter is, my girlfriend at the time was a stupid slut and my "friend" was an asshole. Soon after that, they got entangled into a complicated love triangle. Then I find out that he came in her and made her take a morning after pill. I was watching from the sidelines eating popcorn and fucking another hotter chick. Win-win.
Moral of the story: Don't touch your bro's ho.