How bad of an idea would it be to let my best friend and gf if ten years hook up?

Gf doesn't know I'm into this stuff. She has cheated on me once and I forgave her, which was easy to do. Once, when drunk I confessed that her cheating turned me on and I would like her to get with other guys. She took it well, but didn't say whether she was into it or not, she needed to think about it. The next day I was embarrassed, so I lied and told her I was just hurt from her cheating and trying to cope and wasn't actually into it. That was like two years ago and as far as I can tell she believed me and hasn't brought it up at all since.

My best friend was my roommate for years, and he has always been into my gf. Not romantically, but he definitely wants to fuck her. He used to get drunk and literally say shit like "I'm gonna fuck her someday" and tell me that he just "had a feeling" that he and her would hook up. He'd also just constantly talk about how nice her ass was. It used to cause a lot of fights between us.

It used to really piss me off, but for some reason I started thinking about it every time I'd jerk off. There were a few moments that I thought it might even happen. Once we were all hanging out, all super drunk, and he and I were kinda play fighting. My gf joined and started attacking him with me. He ran to the bedroom right as the pizza guy knocked on the door. I went to get the food and came back and my friend and gf were like grappling in his bed, her on top straddling him and pinning him down, laughing. I told them the pizza was here and they stopped immediately, and didn't seem guilty or anything, but I got the vibe that there was some tension. It got me wondering what would have happened if I hadn't interrupted and had just waited to see how it would play out.

Anyways I didn't mean to make this post this long, but the gist of it is, I hear people say all the time that you shouldn't let your friends fuck your girl. But it's my biggest fantasy, especially since he used to say he was gonna fuck her just cause he knew itd piss me off. He'd only say that shit when he was drunk, and always apologized, for the record.

So yeah should I wait till there's a moment that feels right and just see what happens?