Need some help

I've been lurking for a while and this is a great site! But, I need you guys help.
I've never been cucked, but am obsessed with doing it. I have the perfect situation that I'm having trouble capitolizing on. My GF has a ex-fuck buddy that lives two hours away, but comes into town every couple weekends. He still calls her to hook-up but she won't do it because of me, he also doesn't know about me. He has a much bigger dick than me, I'm average, and he used to just come over roughly fuck the hell out of her and then leave. We've talked about her fucking him again and I think that she wants too, but she can't understant why I would want her too. Basicly I don't understand either, I just know that I REALLY want her too. She thinks that I want her too so that I will be able to hook-up with other girls. She also thinks that if I want her to do this that she and the relationship really don't mean much to me. Which isn't true. We were very close to doing it, she told me all about how he fucked her, his big dick, how she wanted me to eat and fuck her afterward. Talked to him about coming on the weekend or going to see him. Then she wanted me to explain why I wanted her too. I told her I wanted her to be satisfied and that I didn't think I satisfied her and if he could then I was ok with it. I told her how much it turned me on and that I wanted to ravage her body afterword. It wasn't enough and she called the whole thing off. We still talk about it some, and the same explanation isn't enough. I need help explaining to her why I want this and that it won't jeoprodize the relationship? Please help with some experiances or advice. I've always been turned on by this. I'm not submissive, don't have trouble satisfying women except for her, its a first.� How can I convince her when I don't know why myself? Help me understand and make her understand.
Thanks!