Yep done it many times.
It's quite a common occurance that I'm propositioned either by husbands or wives if I'd fuck the wife in front of their husband. Particularly when attending nude resorts or on the swing scene is very common to meet couples who have a cuckold relation ship or fantasy or just a couple who are fascinated to try something bigger than their husband.
Similar experiences but with an important twist.
My first wife and I married very young. Her family were a bunch of nudists. We often went with them to nudist/camps. My wife was very cute. I'm good-looking but hung like a chipmunk; soft, nearly buried in my pubes. We quite often got approached by men, usually well-hung, to fuck her while I watched. We were too naive and uptight in the early years of our marriage to do that. Later on, when we were both having sex outside the marriage, it didn't happen because my wife wasn't into that type of thing.
During the latter half of my marriage when we were seeing others, I and a girl I was going with had a series of fourways with another, slightly older couple (early 30s vs our early mid 20s). One night, while his wife and I were watching him bang my girl, she commented that her husband was in seventh heaven; he really got off fucking smaller-hung guys' wives. That was his favorite thing.