Sex is shorthand for sexual intercourse and other forms of sexual stimulation. We also call males and females the two sexes.
Though sex is often considered a dirty topic, it's pretty important: without sex, there'd be no people, or animals for that matter. There are many types of sex, and the word sex has a few meanings. "Male" and "female" are considered the two sexes or genders, while farmers who sex their chickens are figuring out whether they have a rooster or a hen. Sexing can also mean sexually arousing or stimulating. We hope the farmer saves that kind of sexing for his wife.