Welcome Gheldon...

I wouldn't lose any sleep if I were you.. your GF left you for your best friend.. at that point.. all bets were off.. There is absolutely no reason why you shouldn't sleep with whomever you wish as long as it was consentual.. and it was. If anything, I guess I would have to ask you about the friendships you keep. It just doesn't sound like there is alot of loyalty to be found in that bunch. As a friend, I consider any man off limits that a friend has been seriously involved with (unless they tell me before hand to go for it). I would imagine that in most cases that is typical between friends and I must wonder why not only did her girlfriends not live up to that... but either did your best friend. Maybe your solution in part is to distance yourself from the lot of them and build friendships with people who put more importance on loyalty.