hey Dannyboy !

IT IS a bit of problem !!!

Well, i will say you're young so i guess your gf too, some of us started their "real life" (yeh, this one with BJ) around your age ;
so, first i'd like to say that maybe she loves the band and you don't know it, she didn't say anything to u for the same reason as yours ; maybe she's not a fan actually, but if someone help her to know Bon Jovi, maybe she'll be the first to buy the new cd ... like so many people on earth ! and even if she won't, no problem ... she can buy anything she likes, during the moment she's don't making fun of you or anything bad or negative about your life and wha u like!

My husband loves them, he's not a fan, absolutely not, but he came with me last time to Paris Bercy to see them and he had a good time, really, i play BJ music every day at home and that's not a problem, he can understand ; i don't really love the artists he loves (in music or cinema), but that's ok, then, i think if two persons are happy to be together, music or wathever shouldn't be a trouble, of course, respect, tolerance and make concessions sometimes ;
But if this subject begins to "product" troubles between you, then you'll have to think about what you'd better do ...

I think you're not alone in this situation, and except extreme situations like : BON JOVI are so old .../ or WHAT ??? you ??? you love this band ???/
well, i think and hope nobody on this planet can say or even think that

Good luck, do what you have to do