I would say that it is not that we want essences in our sexuality, but that gender and sexuality are essentialist by nature: the sexual drive is built on top of the parts of our brains that essentialize/abstract/encapsulate, and so reducing the concept would involve modifying the human utility function to desire the parts, rather than the pretended whole.

Or, to put it another way: a heterosexual blegg is not 50% attracted to something with 50% blegg features and 50% rube features; it is attracted only to pure rubes, and the closer something is to being a rube, without exactly being a rube, the less attractive it is. This is basically the Uncanny Valley at work: some of our drives want discrete gestalts, and the harder they have to work to construct them, the less favorably they'll evaluate the things they're constructing on.