Maybe, instead of YOU talking to her - you should see a counselor together? I'm thinking it's too easy for her to push it off on you - you're too horny, too needy, too pushy, whatever. Maybe in front of a third party she'd have to deal a little more with why and how she's withholding from you?
Now - on the flip side - how's her health? Does she have any medical issues that would make her fatigued or just not that into it? Does she take any medications, have a history of depression or thyroid disease?
And how's the rest of your R? Does she have any reasons to be holding onto resentments towards you? Does she have any past sexual abuse issues?
I agree with the other posters, I wouldn't marry her until this is worked out; it's not likely to get better otherwise. And sex in a marriage should be joyful, not fraught with tension.
