a) she shouldn't use sex as a manipulation tool. If she doesn't want sex anymore, you should discuss the future of your relationship because perhaps it's time to part ways prior to marriage. If she wants sex and is withholding it from herself, she's being stupid.
b) The real issue is she wants to get married, like most women. The problem is, most of them don't want to worry about the details, just "make it happen". Since you obviously have a number of other costs and priorities, you need to get her involved in the details. Say, we can prioritize marriage as #1, but which of these others do you want to sacrifice? She needs to make the call, so make her. If she says she's OK with not sending the kids to collage or fixing the house, she will have to own it. There is no reason for you to make the difficult decisions and deal with the stress on your own. You'll grow closer by dealing with it together.
