Sounds like you're trying to change a lifetime of what is now habit. It will take a lot of time, but I would say bring it up now and again without trying to push it. If you can as a "gag gift" buy her some sexy lingerie. Perhaps making a joke out of it will make the subject more comfortable, and then subsequently eventually more considerable. I'm just spit balling though and it's based on what's been discussed in my presence by people older and wiser than me.

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"BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Will he offer me his mouth? BOY: Yes. GIRL: Will he offer me his teeth? BOY: Yes. GIRL: Will he offer me his jaws? BOY: Yes. GIRL: Will he offer me his hunger? BOY: Yes. GIRL: Again, will he offer me his hunger? BOY: Yes! GIRL: Yes. BOY: On a hot summer night, would you offer your throat to the wolf with the red roses? GIRL: Yes. BOY: I bet you say that to all the boys!" -Meatloaf