How do I get my wife to cuckold for me??

My wife gets very turned on by the thought of having sex with another man while I watch. We fantasize about it all the time, and when we do she gets soaked, VERY aroused and very hot. I can get her to the point where she is screaming to me that she wants me to watch her fuck another guy. The problem is after we are done, (and she has orgarmsed) she completely reverses her opinion and decides that she is really not wanting to do this, she just gets turned on by what we are talking about? I really can't figure it out, because I have told her on numerous occasions that I would get very turned on watching her do this.

I am dying to see my wife with another guy, so I wonder if any of you have some advice for me on how to get her to go through with it?

Thanks
Wshrig