As an impressionable adolescent, I heard a disk jockey announce a song by an exciting new talent whose name was not recognizably of a particular sex. I loved the song and guessed that it was done by a woman, with a man singing bass on the chorus. Shortly thereafter, however, I learned that Elvis Presley was male.
Kenan and Kel, the stars of the movie Good Burger, surprised me by both being male, contrary to the impression I'd gotten from the movie posters.
Somewhere between those events, but probably closer to the latter, I became aware that in the culture I inhabit, one is supposed to have a Sense of Gender Identity, one that happens to coincide with one's own genitalia.
I don't. I have male genitals, and almost all the people I find sexually desirable have female genitals, but I don't have a Sense of Gender Identity.
As I say, I think this is one of the things my parents got right, like being less insane about race than most white Americans in the 1940s. Nevertheless, this Gender Identity business is one of the things that make me feel like an alien of one sort or another. People take this stuff seriously. In the 1960s there were those who complained that men wore their hair so long that one couldn't tell which sex they were. I wondered what they really meant by that, but I have come to the alarming conclusion that many who said this did not have some hidden agenda; they really found it seriously important to know the sex of every passing stranger.
I don't. At one job, I saw, but never spoke with, a flat-chested person with a crew cut. I was genuinely surprised when I saw this individual enter the women's room. I guess I felt a bit uncomfortable. What I certainly didn't feel was that I had any right to tell her to dress differently or otherwise keep from misleading me. That would be as stupid and invasive as . . . as saying out loud that a stranger I saw on the street weighed too much. (Oh, that's right; people do that too.)
In Susie Bright's Sexual State of the Union there's an essay, called "I Enjoy Being a Gender," in which Bright says that sexual oppositeness is something she finds essential to sexual pleasure--not opposite genitalia, but one butch and one femme. (She prefers to be the latter, but that matters less.) This is important to her; she even slips from her usual tolerance to stigmatize the opposite approach with the therapeutico-moralistic term "narcissism."
There are other gender approaches: Some men are omnifutuent (a useful term I learned from Walter Breen the one time I met him): They don't care whether their partners are male or female, as long as they get to be the convex one. Senator Joseph McCarthy apparently was one of these, and Gore Vidal's autobiography suggests that that he is another.
(Further evidence suggests that Vidal has been known to cheerfully settle for external friction, as did the late Malcolm Forbes. I did not think much of Forbes during his lifetime. He published at least one book of his own wit and wisdom, and it struck me as the work of a rich man who never noticed that the ones who found him amusing were employees and debtors, or those who wanted to become one or the other. As soon as he died, the closet door opened, and we learned that he was bisexual. One male would-be partner reported that Forbes rubbed against him and said, "It's a man's duty to have as many orgasms as he can-with women, with men, with animals, anything." I have to admit I find that funnier than any of the alleged witticisms in his book.)
Then there are those who pride themselves on being tough and masculine, but want to be taken and penetrated by someone even more so. This would seem to include Roy Cohn and Ayn Rand. The infamous Andrew Sullivan "PowerGlutes" ads that caused so much controversy state that: 1) he is particularly virile and muscular; and 2) he wants to be the concave one. It is also suggested as the female ideal in the works of John Norman.
This could all be fun, but people take it too seriously. Everyone should read Clifford Geertz's essay "Common Sense as a Cultural System" (in his collection Local Knowledge). It's full of good stuff (even its title says things that more people should know). Geertz writes about the fact that in any human population a few infants will be born with genital conformations not immediately identifiable as male or female. There are, he reports, three ways cultures deal with this: Some treat the people born that way as holy and give them positions of importance; some treat such people as the Creator's mistakes and condescend to them; and some find the concept so horrifying that they try to pretend it never happens. Geertz is so offended by the third group that he breaks one of the most powerful tabus of the Anthropologist tribe: He calls them "savages."
About 50 years ago, there was an sf story cast in the form of a condescendingly anthropological study of a primitive tribe called the "Nacirema" ("American" spelled backwards), and one can find much to condescend to in that group. The savages Geertz discussed are of course the Nacirema. One of the ways they try to deal with this frightening business is that the doctor who delivers a child with anomalous genitals is empowered to cut off the parts he thinks don't belong. (An Ob/Gyn who read this essay in a zine informs me that there is an increasing trend to involve the parents in this decision. On the other hand, a recent study suggests that approximately five times a day in the United States, surgeons change the size and shape of a child's healthy clitoris.) That's savage, to say the very least. These doctors would appear to be even more presumptuous than the macho man down in Texas who killed his son because the boy was showing signs of incipient homosexuality. At age two.
Instead of everybody being one or the other, we have a situation where the overwhelming majority are one or the other, but there are enough exceptions that any statement about "all men" and "all women" has a certain irreducible fuzziness. This also means that any attempt to use the male/female distinction can be no more precise than what it is based on. And male/female is about as close to an absolute distinction as we get in dealing with human subjects. It is obvious that if we were to try to graph Black/White (itself an oversimplification of race that leaves out the various sorts of people known as "Asian"), we would not get anywhere near as close an approximation to two utterly discrete piles.
And if we want to determine hetero/homo, we immediately start with imperfectly defined sets of partners. In the mid-60s, I was working in the War on Poverty in San Francisco. There I met John and Joe (not their real names, which I have long since forgotten), who were lovers, and thus presumably homosexual. But (and this was strange and different at the time) Joe was about to undergo sex-change surgery. So John was then homosexual, but in the future, when he loved Jo, he would presumably be het, thus becoming an entirely different person (by the rules I then believed) without changing at all. This did not compute. I later generalized this idea to Schrödinger's Blowjob: Imagine receiving and enjoying excellent oral sex silently, in the dark, so one has no idea of the sex of one's partner. Is the person so blessed homo, het, or bi? Discuss.
Pat Califia's book Sex Changes gives a discussion and history of a lot of these questions, from Christine Jorgensen's pioneering sex-change surgery in the 50s to today, when surgery is better (though it still has a long way to go) and some people are deciding that they can change their gender without changing their sex.
But as Califia points out, this is dangerous. One often hears that women are allowed to dress as men. Actually, they are allowed to do so only as long as they don't fool anyone. The penalty for being caught fooling people (that way or the opposite) can be death, and Califia offers case histories. Personal disappointment doesn't even have to be a factor. An EMS crew in Washington, D.C., stopped working on an accident victim and started making jokes when they noticed the male genitals under the dress. (The victim survived and sued the city.)
I've got some idea of why this goes on: fear of the unknown, advertisers making money off fixed sexual images, resentful white males who've been judged on their abilities rather than their race and sex wanting somebody to feel superior to, the desire to have an easily explainable two-valued world.But there's got to be some way to stop it.
I have always felt that "masculine" and "feminine" are no better to describe types of behavior than "Caucasian" or "Negroid." This again runs into good old Evolutionary Psychology: All other mammal species have fairly strong differences between males and females, so why shouldn't humans?
It is certainly true that human beings are animals, and as animals have the animal traits, including sexual dimorphism: The bucks want to spread their seed as widely as possible, while keeping other bucks out of their territory, and the does want to have lots of cubs and find a big, strong buck to protect them. But surely we have other characteristics, mental and social. I suppose it isn't possible or desirable to completely transcend our animal heritage, but I am in favor of making it less important.
If nothing else, it seems counterproductive to go to heroic efforts to change a successful person who has the "wrong" good qualities for his/her genitalia. C.P. Snow, hardly a figure of progressive enlightenment, pointed out half a century ago, "It is one of our follies that, whatever we say, we don't in reality regard women as suitable for scientific careers. We thus neatly divide our pool of potential talent by two."
Besides, there are other consequences to the evolutionary approach. Eric S. Raymond, among others, has mentioned that if the buck is going to spend a lot of time out fighting other bucks, then the doe might wish to optimize her reproductive strategy by second-sourcing the needed sperm, especially if she can do so without the Alpha catching her. There appears to be DNA evidence that more of this than we might think goes on in other species.
Another note on reproduction and evolutionary psychology: Martha Hrdy has written a book pointing out that the animal model of mothering does not lead to desperate efforts to preserve each and every offspring one has had or is about to, a fact that should be obvious to anyone who has seen a mother cat eating her imperfect kittens. In particular, this means that a woman might have an abortion as part of a successful reproductive strategy. (Ursula K. Le Guin has written a fascinating memoir of doing so.) 
*Pat Califia changed sex since I wrote this and is now Patrick Califia.