How does transvestism challenge the "two-sex" model?

The two-sex model is a way of dividing people into categories based on male and female biological differences. The two-sex model was first theorized in the 18th century; popular opinion beforehand lay with the one-sex model, which stated that women were less perfect versions of men. Transvestism does not necessarily challenge the two-sex model. Transvestism is the practice of dressing and acting in a style or manner traditionally associated with the other sex. Sex and gender are not synonymous. Sex refers to whether a person is biologically male or female. Gender is a state of mind, and its boundaries are not as rigid as sex. One can be born female, but live as a male. Though a man dressing as a woman does challenge traditional gender roles, he is not modifying the biological state of his body. If someone is of the third gender, they are considered neither male nor female, which challenges the two-sex model, but this is not to be mistaken for the third sex. Typically, those of the third gender prefer to be referred to with gender-neutral pronouns such as one, ze, sie, hir, or ey.