Or gender (if any) will come to mean something very different from what we're used to. Once humanity starts bioengineering itself, the range of possibility opens up tremendously.

You mean there are people who don't just shape-shift their apparent sex depending on the precise situational combination of their current mood and what they want to signal? Goddamn you lot are weirdos ;-).

Although the conservation-of-mass issue gets awkward: you can either be a scrawny male but a healthy-looking female or a healthy-looking male and a rather overweight female, or be kinda awkwardly lithe in either form. Or you can go to the bathroom and store the extra unwanted mass when female, but really, there's only so much biological nanotechnology can do against basic physics.

(EDIT: Yes, I did make this up on the bus one day when thinking of things future people might consider good ideas that we'd consider unutterably bizarre, and was indeed waiting for an opportunity to post it ;-).)