I think perhaps the most Marxist way to put it is that gender and sex form a dialectical relationship that cannot be fully understood as two enjoined spectra but only through the lives of real people and the way that they interface and feel best interfacing with society. For my wife at least, her understanding of gender and sex was radicalized largely by trying to understand myriad indigenous understandings of gender, and perhaps that's a good inroads for a certain kind of "well-meaning" liberal.