I think the fear isn't that everyone of the preferred gender wants to bone your partner, but that you have no way of knowing which ones do. The uncertainty is what I think gets to a lot of people.

Still, it's silly. If one of the core values of your relationship is that you're exclusive, it's up to you and your partner to honor that, not up to every other person who comes along. If your partner won't respect that if a friend offers, then they don't respect the relationship to begin with and you're better off finding out.

At least, that's my take from listening to other people. I'm not a sociologist or anything.