Sex (trait)

Biological sex was a physical trait of many races, including humans (TV: Doomsday) and Time Lords. (COMIC: A Little Help from My Friends)

Humans
The Cybus Industries Cybermen of Pete's World sought to remove human sex to become identical. (TV: Doomsday)

Human women were sometimes regarded as the "fairer sex" (TV: Carnival of Monsters, The Time Warrior, The Ark in Space, The Talons of Weng-Chiang, Twice Upon a Time) while human men were regarded as the "dominant sex". (TV: The Time Monster)

Zoe Heriot, being from the late 21st century, found England in 1900 to be "barbaric" and "the primitive culture". She cited the cultural practice of the two sexes separating after dinner, where women went to the drawing-room and men went to the smoking-room. (PROSE: Foreign Devils)

By 200,000, "ladies, gentlemen, multi-sex, undecided or robot" were distinctly recognised traits. (TV: The Long Game)

Time Lords
Sex was flexible for Time Lords:

"Biological sex is flexible among my people, and gender is merely a social construct."

- Thirteenth Doctor

Time Lords sometimes dreamed of being the opposite sex. (PROSE: Human Nature)

Other species
The Saraani were a species with only one sex, neither male nor female. They had no gender identity and accepted whichever pronoun a speaker felt most comfortable with. The reproduced asexually. Vilbian found it incredible that some races had two or more sexes. (PROSE: Dry Pilgrimage)

A researcher believed Question Innotation to be an example of "gender reunification", where an organism divided into two sexes (male and female), only to converge and form a "common mind" gestalt that resembled a single being.

Peri Brown had a hard time telling male and female Eknuri apart, but realised she was only assuming they had two sexes. As far as xenologist Aline Vehlmann knew, Eknuri didn't change sex. (PROSE: Superior Beings)