:: sexual activity between people / physical activity between people involving the sexual organs // sexual intercourse
Her mom talked to her about sex.
She doesn't like all the sex and violence in movies.
Most young people now receive sex education at school.
She'd been having sex with a colleague at work for years.
The sex scenes had been cut out of the English version of the film.
Sex before/outside marriage is strongly disapproved of in some cultures.
She's always dragging sex into the conversation.
Later on in life the sex drive tends to diminish.
The magazine, which gives frank advice about sex and romance, is aimed at the teenage market.

casual sex (= sex with someone you do not know)
unprotected sex (= sex without using something to prevent disease or becoming pregnant)
extramarital / premarital sex (пазашлюбны / дашлюбны)

shag (verb) — a rude word meaning to have sex with someone, a British slang term for sexual intercourse (UK offensive)
shag (noun) — a rude word for an act of having sex, or for a sexual partner (UK offensive)

:: the fact of being male or female (пол)
Do you know what sex the baby is?
The couple didn't know what the sex of their baby would be.
What sex is your cat?
It's illegal to discriminate against people on the basis of (their) sex.
sex discrimination

the female sex — жаночы пол
the male sex — мужчынскі пол
the opposite sex — процілеглы пол
Members of the opposite sex are not allowed in students' rooms overnight.

секс, пол