I agree...it's a difficult situation to maneuver in. This is the same girlfriend who was very opposed to me learning to ride last year, but has been very supportive on the whole. Her enthusiasm has yet to reach my own, and any desire to ride as a passenger is non-existent. I'm hoping over the course of the next month, as we see more motorcyclists on the road, I'll be able to introduce the idea to her familyThis could cause untold grief. Deceit is not an approved behavior, and if the girlfriend is in on it, too, it's worse. You will eventually have to make a choice: ride and be your own person, or be a puppet of the gf's family, behaving "properly."
Though if somehow before then the lie comes out, there will be much 
