From where I'm sitting I think that you should absolutely bring it up. But not just with your gay best friend (gbf) but also with your ex. I'm guessing it was your ex who told you which I can only presume had to be the oddest conversation in your life. I feel like your ex owes you some answers since, well, he's clearly gay or at least bi which I assume you didn't know. Cheating is one thing and definitely signals betrayal but going the extra step of engaging in gay sex while he was with you - that might make me flip my sh*t because I might feel like I was completely lied to the entire we were dating.

Basically, dem's fightin' words.

As far as your gbf, what a douche. Not only was he keeping a secret from you (your ex's activities that were clearly wrong) but he was enabling them behind your back. This guy cannot still be your best friend. You have the right to ask him what he was thinking and why he would do such a thing to you. I also think you'd be well within your rights to throw things such as those little edible fish cracker things. Or little green army men. Nothing that might maim or kill but just something to get the point across.

The other reason you need to bring it up is because you have no idea how much OTHER dirt he's done to you. He might say that sleeping with your ex is the only thing he's done to you that was wrong and I wouldn't believe him for one second.

I say clear the air, and keep it moving...without him.

It was written.