性別
せいべつ · seibetsu · JLPT N2 · noun
sexual activity
性 (nature, sex) + 別 (separate, tell apart) = the nature that tells the two sexes apart
性 means nature or sex, and 別 means to separate or distinguish. Together they name the distinction between male and female, used for both sex and gender.
性 — heart radical 忄 + 生 (life/birth)
性 is a semantic-phonetic compound. The heart radical 忄 supplies the meaning, pointing to inner feelings or innate qualities, while 生 ('birth, life') gives the sound and hints at what one is born with. It originally meant 'inborn nature' and later extended to 'character,' 'gender,' and 'sex.'