You talk about it before everyone is in an amorous mood. If anything, the process of discussing what the night will look like and what everyone does and doesn't like can very easily set the mood.
This happened to me just once. I was OK with it. It was actually interesting to have another guy suck my cock. As we were still working on my wife I didn't cum. But this guy, probably because he's a guy, had a much better technique than most women who have sucked my cock