In the western world, the earliest works depicting homosexuality come from the ancient Greeks, where the practice of adult men having sexual relations with male youths was considered quite normal. The term lesbian dates back to the Greek poetess Sappho, born on the island of Lesbos between 630 and 612 BC. Plato (427 to 327 BC) praised same-sex relationships in his early writings.

In Europe during the Renaissance, homosexuals fell increasingly under the strictures of religious courts. In 1494, the Dominican Girolamo Savonarola of Florence, Italy, declared sodomy a capital crime. Since then, the clergy and legal authorities have declared homosexuality everything from a mild perversion to a sinful abhorrence. In a handful of Middle Eastern countries, homosexual activity is still punishable by death today, though most western countries have at least decriminalized it. It took until 1973 for the American Psychiatric Association to remove homosexuality from its list of mental disorders.

Yet, despite the legal, spiritual and social consequences of homosexuality, more than 2 percent of American men 14 to 44 years old defined themselves as homosexual in the 2002 National Survey of Family Growth, and some experts say the total numbers of gay, lesbian, bi-sexual and trans-gendered Americans could be as high as 10 percent of the total population. By any measure, those percentages translate into considerable numbers: somewhere between 6 million and 30 million individuals in the U.S. alone.

Theorists have debated the question of whether homosexuality is inherent or caused—nature vs. nurture—for decades. In 1945, Alfred Kinsey published "Sexual Behavior in the Human Male," which rocked the world with its revelation that about 10 percent of his subjects had experienced sexual relations with another man. Subsequent studies by other researchers revealed no correlation between development or illness and homosexuality, leading the APA to declare in 1994, finally, that "... homosexuality is neither a mental illness nor a moral depravity. It is the way a portion of the population expresses human love and sexuality."

In the 1990s, at least three separate studies revealed differences in homosexual brains. Along with Dean Hamer's potential discovery of a "gay gene" in 1993, the argument that homosexuality is naturally occurring is strong.

Studies into animal sexual behavior support the human research. A review of existing research by scientists from the University of California, published in Trends in Ecology & Evolution in June, concluded that same-sex behavior is "a nearly universal phenomenon in the animal kingdom, common across species, from worms to frogs to birds," reported physorg.com, a science and technology news service.

Social theorists also have arguments as to the reasons behind homosexuality. In the 1970s, Frenchman Michel Foucault espoused the theory that homosexuality is a social construct, something made up to justify aberrant behavior. Others believe that conditioning and environment—unresolved Oedipal conflicts á la Sigmund Freud and weak opposite-sex relationships—can cause homosexuality. Conservatives embrace social theories of homosexuality, believing that if the behavior is caused through conditioning and environment, changing one or both can "cure" it. Little evidence exists to support the premise.