The Ian M. Banks quote equates gender and sex. That's probably a product of when it was written (1994) and I don't think too badly of Banks for writing it, however, in the contemporary dialogue, these are not equivalent terms. I caution you against getting caught up in an uninteresting semantic argument here: the question isn't one of definitions, but rather whether the phenomena classified as gender (clothing, hairstyles, etc.) are or should be tied to the phenomena classified as sex (genitalia and/or chromosomes). I won't comment my opinion on that as I don't have the time to do it justice, but I will say that to ignore the fact that those phenomena are very much in debate, misses a pretty big point:
That point being, that you can actually change your presentation of gender with relative ease. Norah Vincent did it (dressed as and pretended to be a man) for a year and wrote the book Self Made Man about her experience. The fact that the majority of people never change their presentation of gender and those who do change their presentation tend to feel strongly about it, says that there's more complexity to the choice of how to dress or style your hair than an economic cost/benefit analysis. The fact is, people want to be their gender even while claiming that their gender is treated poorly, and in the case of trans people, choosing to present in a way that is obviously treated poorly. If the hypothesis that people will gravitate to the gender presentation which is treated most positively were true, few people would choose to be the gender that gets spit on in the grocery store (happened to a trans friend of mine).
That point being, that you can actually change your presentation of gender with relative ease. Norah Vincent did it (dressed as and pretended to be a man) for a year and wrote the book Self Made Man about her experience. The fact that the majority of people never change their presentation of gender and those who do change their presentation tend to feel strongly about it, says that there's more complexity to the choice of how to dress or style your hair than an economic cost/benefit analysis. The fact is, people want to be their gender even while claiming that their gender is treated poorly, and in the case of trans people, choosing to present in a way that is obviously treated poorly. If the hypothesis that people will gravitate to the gender presentation which is treated most positively were true, few people would choose to be the gender that gets spit on in the grocery store (happened to a trans friend of mine).