We currently have two questions tagged sex that talk about behaviours that are possibly sexually motivated and three questions tagged gender-differentiation.
My first instinct when seeing the tag sex was that it is about gender differentiation, and I personally find gender-differentiation a bit unwieldy and not necessarily the one I would search for when looking for questions in this regard.
I would probably tag a question about sexual behaviour with sexuality and behavior and one about gender differentiation with sex or gender but your mileage may vary, so I wanted to get some community input.
Note that gender-differentiation at the moment at least has a tag wiki excerpt, whereas sex has not. So we should at least provide one for the latter in case we decide to keep them as is.