Lesson One, Culture:
Cultural Anthropology: Cultural anthropology is the study how people who share a common cultural system organize and shape the physical and social world around them.
Culuture: the customs, arts, social institutions, and achievements of a particular nation, people, or other social group
(So, these two entries highlight that culture is defined by the society that holds it. Science, specifically biology, is not a factor.)
Lesson Two, Biology:
Biology: Biology is a branch of science that deals with living organisms and their vital processes
Reproductive Biology: Reproductive biology involves the scientific study of the reproductive system. Improved understanding of reproductive biology can lead to new treatments for reproductive disorders, such as infertility.
(So, with these two definitions, it shows how regardless of culture, the language of body anatomy is universal. While not all-encompassing, the focus is to define and break everything down to as many boxes/classifications as possible.)
Lesson Three, Putting it in context:
Cultural term:
Gender: the male sex or the female sex, especially when considered with reference to SOCIAL and CULTURAL differences RATHER than BIOLOGICAL ones, or one of a range of other identities that do not correspond to established ideas of male and female.
Biological term:
Sex: either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions.
In short, a culture can say there are as many genders as they like. However, biology shows us that there are usually one of two sexes within the homonid species.
IN SUMMARY:
If you want to mock those who are transgender, at least do it with the right approach, however faulty: "There are only two sexes."