Sex is for the marriage bed but its not too late to wait. That's about all I'll say on that.
"Are" and "should" are two very different words. There's also a passage that if a man (or a woman) is married to an unbelieving spouse, they shouldn't leave their spouse over it, but show God's love to them so that they too will believe. In an ideal world, spirituality would be a much more important factor in choosing a mate. Its your faith - what you would pass down to your children, what your friends and family would see you as and be inspired by you from.
We talk about it from time to time because we have an ideal set for ourselves that revolves around God being first. We challenge each other to be better followers of Christ - to be honest, God
really is most of what we talk about. What He's done for us, various books of the Bible, witnessing, His plans for the family, etc...
If you're not on the same page as your SO, either they need your help to catch up, or you need to turn back a few chapters to get back to them.
I am decidedly NOT saying you should break up with your girlfriend - I say, you should pray for her, and let her know you are. You guys need to have a serious conversation about both of your faiths, because it should be at the forefront of the relationship.
PERSONAL STORY TIME IS A GO:
I once had a boyfriend who I almost married. He said he would not marry me unless I became a Christian, which, I did for him. We were "Christians", yes, but we might as well have both been heathens for all the things we were doing between Sundays. Or even on Sundays.

It didn't make me feel more Christian to know full well that I was sinning. Instead of being in the good soil, I wound up in the thorns and turning away from it (yet again - Thank God for second and third chances

).
Based on my personal experience, I have a pretty good feeling that giving up that lust between you two and you leading the way with a more clearer focus on God might help restore her faith. There are tons of fun things to do with each other that are not "sex: lite". Whatever you do that isn't that - that's the fun stuff. You know the answers, man - you know her better than any of us on this board would. What does she like? What do you like? Have any other couples you could go out with too?