"We have known for at least a decade that hundreds of animal species — including birds, reptiles, mollusks and, of course, humans — engage in same-gender sexual acts. But no one is quite sure why. After all, same-sex couplings don't usually result in offspring. (I say usually because when male marine snails pair with other males, one partner conveniently changes sex, allowing for reproduction.) Evolutionarily speaking, homosexuality should have disappeared long ago.
A yearlong study just completed at the University of California at Riverside offers several fascinating competing theories about why same-gender sexual behavior has endured. And although it's gay-pride month — and the 40th anniversary of the Stonewall riots that sparked the gay-rights movement — not all the theories will give same-gender-loving humans a reason to celebrate.
What all these theories have in common is that same-sex sexual activity is either an accident or a quirky genetic method of helping males impregnate females. Which raises the evolutionary question of why men and women who are exclusive gay and lesbian exist. One answer is that exclusive gays and lesbians are a relatively new creation: the concept of exclusive homosexuality barely existed before modernity; even a century ago, most same-sex-attracted men and women got married and had kids."
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