The main reason why some wives will not fuck with other men is that they are afraid it will mess-up their marriages, no matter how much their husbands protest that it will not. I have never been in the position of having to talk my wife into giving fucks to other men, but if I were, I think I would tell her that I love fucking her and that the fact that other men fuck her, or have fucked her, just will not affect that! And I would tell her that if she let other men fuck her it would make me want to fuck her even more.
What every wife wants to know is that her husband will value her pussy, no matter what, and that he is thoroughly addicted to that hot hole between her legs, and that nothing she does will interfere with his need for her. The perfect words are, in my opinion, "Honey, I would love to watch John fuck you! And when he's finished fucking you, I'll fuck you too! I would love to fuck you while you've got another guy's cum in your pussy! Please let me tell him he can fuck you!"