How do i convince my wife to sleep with a man while i watch

I have been suggesting over the years to my wife that we should have a guy in one of our sexual encounters. I told her that it turns me on to think about her sucking and fucking other guys. Now, she is so damn hot and i know i would love to see her and she knows it. She is afraid that this would mean it would be ok to relax with other women and i have assured her that is not what i am after. When we make love now and for years we she will bring it up in order to get me off.. she will suck my fingers like she is blowing someone or just tell me that it would be nice if there was another cock here for her.. She knows that this is my all time favoritiefantasy.. Finally she asked me if i was bisexual to which i told her no.. She still thinks that i want another man around for that then.. Now when she is talking nasty during sex she will eventually end up telling me how much i would like to suck the dick with her. It is a big turn on and it is not the original fantasy that i had in mind but oh well.. In the throws of passion nasty talk is all welcome.. So, anyone have any experiences like this and if so what did you finally have to do to get your little lady to suck a cock for your viewing pleasure?