<...While 'sex' is a strictly biological matter, having to do with your set of chromosomes and your primary sex organs, 'gender' involves other things such as a person's gender identity and gender-role behaviour...>
I don't see how we can exclude the possibility that "gender identity" is also "strictly biological".
What aspect of an organism isn't, after all?
Besides, the word "sex" is perfectly capable of including all the aspects that some would include under "gender" (e.g. "gender identity" → "sexual identity").
MrP