getting her to dig big cock

My Japanese girlfriend and I have been together for 5 years, a lot of the time spent apart.

We should be getting together permanently later this year. Now, on and off for stretches apart that have lasted for months at a time, we've been getting into lots of phone sex. Last year when I visited her, I gave her a vibrator; and she uses it whenever we have phone sex. Immediately she was able to come; and now she gets right into the phone sex.

our usual fantasy, one I've imposed on her was her fucking another cock; a nice big one, sometimes black, sometimes white. Always big, though, much bigger than my small tool. Even though she never initiates this narrative, she responds to it well. If I ask her if she would really like to try a huge cock, she pants as says she would. After she comes, though, she says that she wouldnt do that, because she cannot equate or separate love from sex. But whenever we engage in that fantasy, she's all for it.

Is she really into the idea of big dick? How do I get her to do it in real life?