The Science of Sex : THE SEXUAL BRAIN, <i> By Simon LeVay (A Bradford Book/The MIT Press: $22.50; 168 pp.)</i>

Homosexuals are. They always have been. I expect always will be. Gay behavior is likely to continue for anywhere from 5,128 to 7.8 million years, a current mathematical projection of the time remaining for Homo sapiens on earth. Yet the controversy over whether some people choose gay lives blazes on. “The Sexual Brain,” by one of the world’s foremost neuroanatomists, further stokes the fires.

Chair of the Institute of Gay and Lesbian Education in West Hollywood and formerly an associate professor at the Salk Institute for Biological Studies, LeVay published his research two years ago in the journal Science. It suggested that a tiny nucleus in the brain--INAH-3--may be smaller in gay men than in straight men.

Now, in this slim and elegant volume, LeVay goes on to define a scientific approach to exploring why we are sexual animals, what brain mechanisms produce sexual behavior and how they differ in men and women. He asks: What determines sexual orientation? Genes, events in the womb or nurture? And in a humanistic style reminiscent of Lewis Thomas’ essays on biology, he invokes the spirit of Shakespeare in chapter headings such as “So Full of Shapes Is Fancy: Sexual Orientation and Its Development.” The melange is engaging and professional--a work of stunning scientific scholarship enhanced by gracious style and modesty.

This is not to say that LeVay is able to prove the biological basis of homosexuality.

Nor does Dr. Dean H. Hamer of the National Cancer Institute in Bethesda, Md., claim that genes on a tip of the X chromosome, recently found to be identical in 33 of 44 pairs of homosexual brothers, prove the genetic origin of sexual orientation. His work, reported in Science last month, intimates that genetic influences may precede and create the brain changes LeVay has observed.

The problem is that, despite the breakneck pace of today’s brain research, we are still nowhere close to charting the interrelationships between about 10 billion nerve cells (neurons) and 10 trillion connections (synapses) between these cells in the brain, let alone the connections between genetic, hormonal, neuronal and environmental events.

LeVay is a stylish, captivating guide, even when helping us negotiate the densest thickets of neuroanatomy. Here, for instance, is his introduction to the hypothalamus, where the INAH-3 nucleus is located:

“People tend to stay away from the hypothalamus. Most brain scientists (including myself until recently) prefer the sunny expanses of the cerebral cortex to the dark, claustrophobic regions at the base of the brain. They think of the hypothalamus--though they would never admit this to you--as haunted by animal spirits and the ghosts of primal urges. They suspect that it houses, not the usual shiny hardware of cognition, but some witches’ brew of slimy, pulsating neurons adrift in a broth of mind-altering chemicals. Let us descend to this underworld.”

LeVay obviously hopes that his research will support the gay activists’ goal of participating with unquestioned equality in all social institutions, from marriage to the military. But in a recent interview in the Nation, LeVay explained that his opponents, mostly religious fundamentalists who are disturbed by his belief that mind cannot be separated from body, find his work “simplistic, threatening, even offensive.”

These people are living in an antique universe, he suggests, where “man’s” upright posture proves that his mind is the godliest, most divine body part, not simply another organ doing its business. If the brain were merely that, where would God find His place in man? Certainly not lower down.

Most of us regard these cavils as anachronistic and irrational. But as a psychiatrist I do see how LeVay’s work raises other dangers. In 1979 and 1986 the American Psychiatric Assn. (APA) removed homosexuality from the catalogue of mental illness. Yet a lesion, atrophy or site of underdevelopment--represented by gay men’s INAH-3--could set us back as a society by showing homosexuals to be “abnormal.” Their mental status could again be questioned, this time on new grounds. At the moment, such a reversal is unlikely. But the threat remains: the APA’s masters of nomenclature rifling their pages, the surgeons sharpening their scalpels. The same threat looms in terms of genetic micro-manipulation to “correct disorder” in the X chromosome.

If biology is destiny, evidence of biological difference gives common cause to gay and feminist politics. Women differ from men by far more than a blip on the hypothalamus. As a consequence, women have historically defined them as dreadful, dark, black, weak, mysterious, menstruating, Other; the dignity of autonomous existence has been at stake. To be other than man is to risk persecution.

Alternatively, feminists often glory in their difference. They heed Luce Irigaray’s injunction to take joy in wombs, fluids, flesh, interiors, exteriors, intelligence, endurance, strength, passion. But only recently have gay men conspicuously taken pride in the gifts of their orientation.

The trendiest thinkers deal with the threat of difference by dismissing it, eradicating the Other and claiming to perceive people as all the same. Logic disintegrates, though, when one asks how, precisely, we can erase the differences between men, women and all their varieties. As long as one degree separates two objects, an Other exists.

One imaginative thesis suggests that sexual ecstasy dissolves difference. This highest transport extinguishes the self, and we are all the same. As Jonathan Dollimore writes in “Sexual Dissidence,” one de-centers the self in the desire for the Other (and loses it in the satisfaction of that desire).

It seems inevitable, however, that ingenious researchers, building on the studies of LeVay and Hamer, will establish a biological source of difference. A time will come when homosexuality will have a physical as well as a psychological dimension, a genetic marker, a locus on the body, a tangible place in the head, the blood, the groin, the heart. The closet will no longer be elective. Outing will be outmoded. But considering humanity’s inability to tolerate difference--our tendency to enslave, rape, pillage, cripple, slaughter and murder--defining this Other at any time may be too soon.

In 1935, Magnus Hirschfeld, the great sexologist whose institute was destroyed during the war by the Nazis, told a story about silkworm moths that may presage some of our own future. In his book, “Men and Women,” he wrote that during his travels in the orient he met a colleague who demonstrated homosexuality in these moths. As the distinguished scholars observed their movements under a watch glass, fragile newborns emerged from their cocoons. Through the glass they could be seen perambulating on their tiny legs.

Maturation proceeded quickly. Soon they mated. An average of 3 out of 100 male moths mated only with other male moths. They ran rapidly away from the same prospect with female moths. These mating males were definitively distinguished from the rest of their kind. They had a tracery of delicate and unique markings on their wings!

So perhaps INAH 3 really grows as a magical tissue to confer the gay mission. Perhaps even more accessible indicators will emerge. Future inhabitants of our planet may quickly be able to tell the difference because, like the finery of the gay moth, obvious characteristics will inform the educated eye. But how will these future Others use their knowledge? What will their attitude be if they are able to observe the marker as easily as we now identify the red badge of mourning for AIDS victims of whatever sexual designation?

And how will gay men, who have historically contributed so much to art, culture, science and ideas, be affected by a signature on their wings? The answers test our common humanity.

BOOKMARK: For an excerpt from “The Sexual Brain,” see Opinion, Page 3.