You only have 2 options, either openly discuss it with her. Or do what I do with all my pets, seduce them with hypnosis.
I have several great hypnosis scripts as well as a simple guide outlining how to seduce your partner with hypnosis with out telling them your going to hypnotize them.
My problem is kind of similar. I'm a sub with a vanilla husband. The furthest he will go is furry handcuffs, and that's it. I have come to an understanding that he wasn't as comfortable with the lifestyle as he thought. I love him, I do. Until something changes, I'll keep having those dreams