Why do married men want threesomes and cannot be satisfied with their wife even if they are good in bed and beautiful?

I would question the Wording, "...cannot be satisfied with their wives..." it makes me wonder if he wants to explore the limits of their relationship. Maybe give her the opportunity to have different sexual experiences within the confines of their relationship. It is difficult for me, without more information, to make the logical jump from saying that he wants a threesome because he is not satisfied.Instead I would look at it form all perspectives and I can see some positives.