In Counterpoint: physiologists should not distinguish "sex" and "gender", Geary spends a while criticizing King's definitions of the two terms, then concludes that since the two terms have been used synonymously since the 14th century, that physiologists should use them interchangeably. I don't think this argument has any merit whatsoever, since languages are constantly evolving and words that meant one thing historically do not necessarily mean the same thing today. It is a fact that sex and gender refer to different things today, and physiologists should not stand in opposition to that. However, Geary does make some interesting points.
[G]ender identity and role result from complex interactions among genetic, hormonal, and social factors. [...] Sadock [...] defines sexuality as gender identity and the associated thoughts, feelings, behaviors, and unconscious neural processes. Gender identity, the person's sense of maleness or femaleness, and the consequences of that, is seen as intrinsically connected to biological sexual characteristics on the one hand and to personality traits on the other. [...] The point is that there is no time when biology is clearly not involved. As described above, biological mechanisms that may influence gender identity are still being discovered.
This line of thinking has interesting implications for the choice vs. genetics argument for people who aren't strictly heterosexual, and while I like where this specific train of thought is going, I don't think it backs up Geary's other argument that we shouldn't differentiate between sex and gender. While they may be biologically linked, they most definitely aren't the same thing. To argue that they should be used interchangeably is a logical fallacy.
King, B. (2010). Point: a call for proper usage of "gender" and "sex" in biomedical publications AJP: Regulatory, Integrative and Comparative Physiology, 298 (6) DOI: 10.1152/ajpregu.00694.2009
Geary, N. (2010). Counterpoint: physiologists should not distinguish "sex" and "gender" AJP: Regulatory, Integrative and Comparative Physiology, 298 (6) DOI: 10.1152/ajpregu.00113.2010