Sex
Sex was the action by which two (or sometimes more) individuals engaged in what was frequently regarded culturally as "intimate contact". Often, this was the means by which they procreated their species. However, as sex was generally associated with positive emotional and physical responses — except when one or more parties did not consent to it, as during rape — sex could also be practised for recreational, and not strictly procreational, ends. Sex was distinct from sexuality, the expression of sex and sexual desire.
Indeed, sex could be practised between two or more individuals of the same gender, which rarely had any procreative implications at all. Precise definition on what specific actions constituted sex varied by species and culture. By the 21st century, humans did not need to have sex to reproduce — the males donated sperm and it was injected into the females by syringe. (TV: Out of Time) Carys Fletcher worked as a temporary secretary at such a clinic. (TV: Day One)
Virginity was the state of not ever having had sex.
Some individuals such as Roz Forrester decided in their youth that sex didn't offer anything that Martian ale didn't. (PROSE: The Also People)
Sex was also a synonym for gender. (PROSE: Human Nature, Something in the Water)
Species
According to the Eleventh Doctor, birds, bees, and even educated fleas have sex. (TV: Hide)
