They used to be REFERRED to as "THE weak sex" or "THE weak gender." Even better, you can say "the weakER sex/gender". I wouldn't say they "belonged" to the weak sex. The whole idea of "the weak sex/gender" is a matter of opinion. By the way, in writing use the word "gender". For obvious reasons.

This is a good question, worth asking! I agree that you will want to say, "women used to be referred to as the weaker sex." What I am more interested in addressing is the question of "sex" versus "gender." Sex is a purely biological characteristic; one is sexually male, or female, or hermaphroditic (a combination of the two). Gender is a little more complicated. It is an expression of sexual identity. American 'women' commonly wear their hair long to appear 'feminine.' Americans like to dress baby boys in blue and baby girls in pink. Boys may be given toy guns, while girls get dolls. American 'men' don't typically wear dresses. But these are typical EXPRESSIONS of GENDER. Obviously either sex can wear a dress, and some men wear dresses, and plenty of women have long hair. Gender and sex are related, but they are not the same. That said, they are often used interchangeably, because many native English speakers don't realize a difference. But a quick note about terminology: As noun or adjectives, 'female' and 'male' refer strictly to sex. Adjectives, 'feminine' and 'masculine' can refer to EITHER sex OR gender. Nouns, 'woman' and 'man' don't refer strictly to sex or gender. :) Hope that was helpful and not confusing!

The term “the weaker sex” is still used. For example, it was the title of a New York Times article in 2006 (The Weaker Sex) and an article in the London Mail online in 2008 (Are women really the weaker sex? The intriguing medical facts that settle the oldest argument of all). When I was young, there were two sexes in humans and three genders in grammar. At that time, gender was a grammatical term, masculine, feminine or neuter, referring to the characteristics of certain European languages. In recent years, the term ’’gender” has replaced “sex” in some uses because, while “sex’’ can refer to the condition of being male or female, it can also refer to the sexual urge or instinct or to sexual intercourse. To avoid the latter connotation (涵义), Americans seem to have adoped the more innocuous (无害的) term “gender” when referring to male or female. I don’t know why. Perhaps some Americans are still sensitive about using terms which have sexual connotations. There is a linguistic principle that goes something like, “bad words force good words out of use.’’ The meaning is that words that acquire an second, undesirable connotation cease to be used in the original sense. For example, “gay,’’ which was once used in the sense of “happy,” is today rarely used in that sense because its use to mean “homosexual” has become widespread. FYI, the term “the second sex” is also used referring to women, as in the book “The Second Sex” by Simone de Beauvoir, a book which deals with the treatment of women throughout history. De Beauvoir was the lifelong companion of existentialist philosopher, Jean Paul Sartre, and a philospoher in her own right.

Since there are only two sexes (unless my early education was completely wrong) women have been referred to as "the weaker sex". This is said by men who don't get out very much at all. The term was popular until about 30 years ago and has fallen into disuse ever since. Technically, "the weaker gender" would be understood, but I've never heard it used. "Weak group" would be practically meaningless. One other way that women as a sex are referred to is "the fairer sex" Saying this won't get you punched or wind up with a drink thrown in your face if you're a man. If you're female, you'll generally just get stared at if you use "the weaker sex.""