She has said that she doesn't want to marry me if she's just my female best friend that I sleep with.

What are her feelings about you? Are you "just" her "male best friend that she sleeps with"? Your post comes across as rather asymmetric.

We both are concerned that I've not really had a relationship not with her, so there are no points of comparison for me to make.

Aren't you "both concerned" that she had too many relationships and so may decide that you are not for her precisely because she has these "points of comparison"? I suspect that she is the dominant partner in this relationship, possibly because she is more mentally mature, and this is often a warning flag.

She sometimes gets mad at me for things I'm "just supposed to know" to do, not do, say, or not say. I'm not sure if she's right and I'm a jerk.

Do you get mad at her for things she is just supposed to know to do, say or not say?

Anyway. DO NOT GET MARRIED YET until you figure out how to be an equal in this relationship (and if you think that you are, then you are fooling yourself).