I don't think its a good idea to get her interested in something you can't offer, because if she finds she needs it more than you thought, where does that leave you in the equation?
This reminds me of being bored at work and finding the literotica.com site, which has erotic stories. I read stories from various section, and started one from the "inter-racial" section. An important note here is that to the writers/readers of that site, black men are hung, white men are not. I found that a comical and revealing aspect, although its definatly what works for them.
Anyways, the stories all revolved around a white man (small penis) wanting to see his wife have sex with a black man (big penis) because of the mans own obsession. In every one I skimmed through, the white lady ended up becoming addicted to the bigger penises, and the husband finds himself crushed since he no longer controls his fantasy. Of course in those stories the black guy usually gets the lady pregnant, she leaves husband, etc.
My point is probably not clear here, but the point is what if your fetish did become something she found necessary in a relationship? Are you willing to throw away your relationship in order to satisfy a perversion?