Two of my friends fucked my wife for years without their wives being aware of it--though one friend eventually confessed to his third wife that he'd been fucking my wife throughout his first two marriages. The other friend's wife still doesn't have a clue, and he's been involved with my wife for 30 years now.
We are old high school chums, and he lives out of state. He comes up to visit three or four times a year, and always expects my wife to give herself to him for hours long sessions. As he is quite the good lay and pretty well hung, she's been happy to accommodate him. Sometimes I watch. Sometimes I participate, but more often than not, I leave them to their own devices. She's much less inhibited when I am not there, and I really get off on thinking she's (actually knowing) she's doing things with him that she's never done with me. We have pretty good sex, but this second friend, well, let's just say he knows how to get the most out of woman.