You shouldn't marry someone who isn't interested in making your sex life good. You're talking about one of the primary distinguishing factors between romantic and platonic relationships and being in a monogamous relationship means you have to rely entirely on that one person to fill that need. If she not only rejects you on the regular but refuses to try anything new, you're in for years of resentment that will fester and boil over into a general dislike or even hatred of her for making you feel so unwanted and undesirable. It will destroy your marriage, guaranteed. Even if you don't divorce, you'll end up as little more than roommates stuck in a weird coexistence that doesn't satisfy either of you.

If I were you I'd put the engagement on hold and make it clear that you're just not willing to walk into a future with someone who doesn't care about your sexual needs and isn't willing to work to make things better. If she puts in the effort and things improve consistently and over a sustained period, then you might be in the clear to proceed toward marriage. If not, then you should break off the engagement rather than resigning yourself to a lifetime of sexual frustration.