Selecting the sex of the PC has been a standard feature in practically every RPG with character creator ever, and definitely in Bethesda RPGs, what they added is an option to change pronouns the PC is referred to by NPCs from the default. If you want them to match you simply don't touch that option, done.

From a developer's perspective it's dead simple, similar in triviality as allowing people to mix+match any voice to any body type. Cyberpunk's free choice of genitals needs some implementation care but if you're planning for it from the start it's also easy.

Where things get more complicated is things like dresses for male bodies, especially if you don't have any shape keys in place. But if you use one body mesh for everything and simply shape key it into male and female then it's again no issue (you also need to lerp animations then, probably, male and female bodies walk differently because hips). Basically it's hard if your asset pipeline is simple, if the pipeline is sophisticated it's easy.