I know seeing this title, people are probably expecting some conservative rant against all the many sexual identities, but I plan to go against the grain and instead rant a little bit about language*.
Way back at the dawn of time -- actually, probably in the era before the second World War and maybe a bit after -- the word "sex" referred to the division of humans and other living things into male and female, while "gender" was used to describe the linguistic practice of dividing words into masculine, feminine and, sometimes, neuter. "Gender" did not apply to living things in any sense other than a linguistic one.
Along come the social sciences, and the academic tendency to coin copious jargon. The field of sociology, and related disciplines, need a way to describe the roles adopted by one sex or the other, even when those roles might be displayed by members of a sex not traditionally associated with them. (Eg. Men wearing clothing traditionally considered female, or women engaging in hunting or fighting, traditionally male roles.) As is often the case, this new area of investigation was at first a bit chaotic, with some using the bland old terminology of "sex roles" and "sexual identity" and such, while others tried to coin new terms to describe these dull old concepts. At length, through whatever process decides such things, the profession -- and allied fields such as psychology -- settled on the use of the term "gender".
In this context, it is usually stated that "gender" is not simply a synonym for "sex", rather it is a specific and very precise term, describing those elements of sex -- or sexual identity -- unrelated to physical aspects of human sexuality. In short, it should be used to describe the societal, malleable aspects of one's sexual appearance and behavior. Sex organs, facial hair and those sort of things are still sexual aspects, not gender**, while clothing, affectations, profession choice and so on, those are aspects of gender.
Now, I am not a great fan of this terminology, as it is basically a needless neologism, or, rather needless redefinition of an existing word. We could do perfectly well discussion "societal aspects of sexuality" rather than "gender roles", but, much as I dislike it, it is not so bad as to be unacceptable. It is somewhat pointless, and a bit confusing since there is a bit of overlap with the linguistic use, but it is not one of those word choices that are so confusing I argue that they should be abandoned.
No, what bothers me about the term is, once it was adopted as this precise bit of jargon, the general public adopted it, but forgot the technical meaning. And so, now we have "gender" being used as a synonym for "sex", and we hear about "societal gender roles" or "non-physical aspects of gender", which are redundant at the very least, if we were to use the formal definition.
And that is my objection to gender, not that the new definition was pointless, but rather, that as a result, we are now saddled with the public tendency to use "gender" as nothing but another word for "sex", meaning we now have twice as many words, but still need to use special modifiers to make clear our meaning. In a way, it reminds me of the war against the word "actress" (cf "A Question About Language"), which led to the silliness of advertising for "female actors", where once one simple word would do. And we are in the same place now with "gender", having popularly made it mean nothing more than sex, we now have to use all those modifiers that "gender" was intended to replace.
* Actually, for me it is hardly out of character, given my numerous spelling and grammar Nazi posts. See "The Most Unnecessary Neologism", "Biggest Spelling Nazi Laugh of the Day", "Quick Grammar Nazi Note", "Return of the Grammar Nazi: Faux Latin Plurals", "Always Something Worse", "Crimes Against Language and Logic", "Try and Listen to the Grammar Nazi", "A Brief Visit From the Grammar Nazi", "Beyond Grammar and Spelling", "The Grammar Nazi Versus George Lucas", "Ye Olde Grammar Nazi", "Grammar Nazi Comment on Greco-Latin Words", "Why Spelling Matters, One More Time", "The Irony of Lax Internet Standards", "Short Grammar Nazi Post" and "The Spelling Nazi Begs to Differ".
** Well, in a way, modern science has made this not quite true. Thanks to hormone treatments and surgery, we can now manipulate the physical aspects of sex, as well as societal. Thus sex is more fluid than it once was.
There is one other aspect of "gender" that has made this term more trouble than it is worth. Since "gender" is taken to be a social construct, people feel content to invent many, many new "genders", adding asexual, transgender and more and more. It seems every minute shading of attitude toward sexual roles is now dignified with being declared an entirely new "gender". This is absurd. One can have a nonstandard attitude toward sexual roles without being a whole new sex or gender. Are we eventually going to get to the point where we each are a sui generis gender? "What gender are you?" "Tom." "Oh, I am Stacey gendered." I am happy people are willing to feel comfortable with their own perspectives, but that does not mean we have to elevate every minor shading of perspective into a "gender". (It reminds me of every minor ethnic group fighting to have its dialect declared a "language". At some point, trying to lend people "dignity" and "worth" can lead to absurd results)