sex(ual)
1.the sex统指“女子们”，是戏谑词语，the sexes指“男子们和女子们”。例如：He never associated with the sex. 和He often talked about the equality between the sexes.
2.a person of the opposite sex里有the，two persons of (the) opposite sexes里可以有the，可以没有the，two persons of opposite sex里没有the。
3.sex（形容词）和sexual没有绝对的区别。除了在sex appeal（性感），sex antagonism（两性间的对立），sex war（两性间的冲突），sex urge（性欲）等少数名词里必须用sex以外，sex不及sexual普通。
下面各词前面用sex或sexual都普通：
- act, attraction, cell, hygiene, instinct, life, organs, pervert
下面各词前面通常用sexual：
- affinity, appetite, behaviour, commerce, copulation, desire, difference,
- emotion, feeling, function, impotence, impulse, indulgence, intercourse,
- interest, morality, perversion, process, relations, reproduction, selection
