sex/sɛks; seks/n 1 [U] the physical activity that two people do together to produce babies or for pleasure 性行为﹐性活动: +with He had sex with his sister's friend. 他和他妹妹的朋友发生性行为。sex education (=when young people are taught about sex) 性教育safe sex (=when people use something that will protect them from sexual diseases) 安全性行为2 [U] whether someone is male or female 性别: We don’t want to know the sex of the baby before it's born. 宝宝出生之前我们不想知道其性别。3 [C] all women considered as a group, or all men considered as a group 男性﹔女性﹔雄性﹔雌性: people of both sexes (=men and women) 男男女女He doesn’t have much success with the opposite sex (=people that are not his own sex). 他和异性交往不大成功。