In my experience a relationship with a woman that does not at least start out fairly passionate is simply not going to last. Stop being such a nice guy. She doesn't appreciate it anyway, even if she seems to. Make a move.
Here is how women think:
They use sex to form bonds in relationships. If they feel like the relationship is threatened they will use sex to try and cement it. In the beginning of a relationship when it's pretty unclear they will use sex a lot to try and establish it, if it's what they want. If the woman is not putting out that means one of two things: (a) she doesn't care whether or not she forms a bond with you, or (b) you've made her feel too secure, so she figures she doesn't have to.
I guess it's possible there's some ultra conservative women out there that just don't put out at all, but I don't really believe they exist. They just pretend to be conservative. They'll put out if you give them the right excuse, becuase that's what women do.
This is also why they stop putting out once they get married--they feel too secure about the relationship, so they don't feel any more need to form the bond by having sex. Let them experience a few weeks of free fall and then bust a move on them and you'll get the best sex of your life, assuming they're interested in keeping you around.