I'm not a guy, but I just wanted to say that I love the idea of watching my husband getting busy with another guy (giving or receiving oral or anal), but not so much the idea of actually sharing him... I don't know if that even makes sense!
I have mentioned to him how much it would turn me on, and while I don't think he's entirely averse to sex with another man, I'm not sure he would ever go for it. He's VERY shy, and even if I had no reservations about it, I don't think he could get past his own.
Still, we've had great sex after some graphic conversations about it.