My husband and I are both open to this and have both indulged.
Example: One of our best friends from high school is one of my fuck buddies right now. We have been for about the last year. He comes over quite often, shares the bed with us, and stays for a few nights usually. My husband is out picking him up tonight, as a matter of fact.
This is just one example because it is relevant to tonight and it tied in with this conversation.
The reason it works is because we are all open and completely honest with each other.