I have a hypothetical situation for you. Say, you have a girlfriend. You’ve been with her for the better part of a year. The two of you are not particularly mushy; you’re both rather independent and sometimes you two are more like friends with benefits, how casual you are around one another. Despite that, there’s genuine attraction between you. You love being around her, you love the time you do spend together, you appreciate the mutual independence, and you love it that she loves you. And as the cherry on the cake, she is fucking GORGEOUS. You’re well aware that you never have a pretty girl alone, though, so you’ve always been wary that you would lose her.
Anyway, you also have a best friend. Your buddy is married but his marriage is quickly going down the crapper. In the past half year your best friend and your girlfriend have become really close friends to the point where it becomes a bit uncomfortable. She is obviously taken with him, and he definitely wouldn’t mind getting into her pants, either.
And there you go. One night after a lot of booze & other substances, they end up in bed together. Turns out your girlfriend loves your best buddy. Your best buddy is utterly horrified when he wakes up the next morning and says that what happened was a mistake.
What do you do? How will your feelings be towards your best friend? Reply! Discuss! Speculate! Please 
I’m very curious to hear.
As far as I’m concerned it’s bye-bye to both, at the very least for a good while. I can understand how such a thing can happen, but it’s practically impossible to move on myself while remaining close friends with either of them in that case. In short: my involvement hampers them and their involvement hampers me, so it’s a good time to split ways.