How to get the Wife to say "Yes"

I've had this nagging fantasy of wanting to watch my beautiful wife to have sex with another guy. We've discussed it for years. Yet, she still resist going through with it for a couple of reasons. So how do I get her to finally give in and put out for another guy?
 
I've had this nagging fantasy of wanting to watch my beautiful wife to have sex with another guy. We've discussed it for years. Yet, she still resist going through with it for a couple of reasons. So how do I get her to finally give in and put out for another guy?
What are the reasons she is giving you?