I just prefer it forbidden. If a husband wants me to fuck his wife without him being in the picture, I will do it as well. My interest is the wife, and that's the only person I wanna have an interaction with. I don't wanna be watched by a husband. Who is to say he can't snap then. When it's forbidden, I am cautious, but also I am prepared for that husband who will snap - I will snap right back.
For me it is fun. I like knowing that the woman knows that what she is doing is wrong, but she doesn't wanna be right. I don't want that warped morality where the husband is too weak to resist his wife's desires, so he goes along, but is silently weeping inside. And then the thrill that you might get caught is fun as well. You can up the ante with places you fuck her. Different people, different desires. I desire the forbidden.