So, you like to have sex with men. Or with women. You prefer handjobs or blowjobs or jelly donuts. It doesn't matter. Regardless of who or what turns you on, your genes remain the same. You come from a long line of successful breeders, and each gene in your body is here because it contributed to the reproductive success of those who came before you.

Before we go farther, some full frontal disclosure: We were asked to write about gay and lesbian sex and monogamy, specifically from a straight perspective. We aren't going to do that. Instead, we're going to write about sex from a different and even straighter view: that of your mother, evolution. She may or may not love you (that's hard to tell), but make no mistake: She made you. Gays and lesbians remain biologically men and women through and through, and evolutionary biology percolates into gay and lesbian relationships. True, evolutionary biology is at a loss to explain homosexual behavior, since genes for non-reproductive sex should quickly disappear, in contrast to genes for reproductive sex. But in any event, every person is born a perfectly good mammal, and our brains no less than our bottoms were configured by evolution through natural selection. So not surprisingly, most homosexual men still act like men, and most homosexual women, like women.

Take monogamy. In nature, sexual fidelity is virtually nonexistent. Biologists used to believe otherwise, but DNA testing has revealed that while some animal species, mostly birds, are socially monogamous, virtually none are sexually monogamous. Even geese and swans cheat. Most mammals don't even attempt monogamy. Why? Lets look at males first.

Sperm-makers spew forth vast numbers of very cheap, plentiful pollywogs, and are endowed by natural selection with bodies and brains designed to spread their sperm about. A squirt costs nearly nothing, and there is little to lose and potentially much to gain by playing fast and loose. And so, males in virtually every species are easily excited. They prefer novelty to familiarity, opportunity to inhibition, sex to celibacy.

By contrast, egg-makers are inclined to be careful comparison shoppers. After all, females are born with a very small number of eggs (compared to the sperm-slingers), and if a woman gets pregnant, she must produce a placenta, milk, and care for the child; so it pays for egg-makers to tell shit from Shinola, to be picky, and to be comparatively slow to warm up. A female who plays fast and loose with her eggs is likely to get saddled with lower-quality offspring, and possibly with a "mate" who isn't available for anything other than mating. For females, therefore, it pays to choose a sexual partner carefully.

Many lines of evidence suggest that humans evolved as a mildly polygynous (harem-keeping) species. Yet several physical traits in humans suggest that females, too, are inclined to have multiple sexual partners. Have you ever wondered why human sexual intercourse involves so much in and out, huffing and puffing? Most animals are quicker and quieter. One theory is that the bulbous human penis is designed to function like a plumber's friend, to dislodge the semen of previous males before introducing one's own. Semen, meanwhile, gets sticky and gelatinous, so as to adhere to the cervix and outlast the gyrations of the next male.

Anthropologists point out that 85 percent of human cultures before the Judeo-Christian homogenization were polygynous. And these days, virtually no one restricts himself or herself to a single lifetime sexual partner, despite the hypocritical rantings of religious conservatives. People date, marry, and divorce, with casual sexual relations and affairs in between. In the straight world, roughly half of men and a third of women admit to having had at least one affair. DNA fingerprinting of randomly chosen children reveals that five to 10 percent were not sired by their "fathers." Monogamy, in short, is a myth for people no less than for animals.

Males of all species are especially poor at monogamy, and we predict that sexual monogamy in gay relationships would be particularly rare. Put two sperm-squirters together, and the likelihood is that one or the other or both--following their biology and perhaps in spite of their "higher" yearnings--is/are going to be unfaithful. We predict that lesbian relationships would be more stable than their gay counterparts, but not entirely faithful, either. Women, too, will cheat, especially if (a) there is an opportunity to obtain serious material resources as a consequence, or (b) the potential third party is exceptionally attractive. At least, that's what females generally do, even in socially monogamous species.

In short, monogamy is not "natural." But that's not to say that it isn't worthwhile. Lots of things that are natural are bad, like acne and pneumonia. On the other hand, lots of things that are unnatural are good, like baroque music. It takes practice, determination, and wit for human beings to do unnatural things, like playing the harpsichord. And some aspects of monogamy may be socially desirable. For instance, monogamy may be a very good way to raise children, if it guarantees two committed adults taking care of the young. This would mean little to gays and lesbians, who are not in the business of breeding anyway, unless you decide to reproduce or adopt and want to plan a stable family for the child. Obviously, monogamy also reduces the spread of STDs.

Monogamy is a good deal for men, too, particularly average Joes who are not wealthy or particularly attractive. In monogamous cultures with equal sex ratios, even middling guys get to have a family. It is possible that this could translate into gay culture; perhaps monogamy is a good deal for people in general who perceive themselves as "average," who would not do well in a sexual marketplace favoring more dominant, more attractive, or more successful individuals. On the downside, monogamy is often less fun than its alternatives, because it restricts possibility without reducing desire. And let's face it, monogamy often equals monotony. Sometimes, its easier to find variety with someone new than to create it with a familiar partner.

Why would anyone who was gay or lesbian even consider monogamy? There is one simple answer: sexual jealousy. As natural as the instinct may be to sleep around, many people have an equally strong instinct to hate it when their partner does exactly the same thing! Monogamy can function as a mutual disarmament pact: "I won't if you won't." Monogamists agree to have fewer sexual adventures, in exchange for less anxiety. Given that a significant amount of violence in the world, including murder, is caused by sexual jealousy, maybe sexual disarmament with a "monogamy treaty" could benefit any relationship--gay or straight--in which parties are prone to jealousy.

At the same time, if you are gay or lesbian, engage in strictly recreational sex either for fun or social bonding, and are not sexually possessive, the advantages of monogamy are debatable. If you crave novelty and have made peace with STDs as well as reproduction, "open relationships" may beckon. The trick would be to find others similarly inclined, and similarly tolerant, so that deception and jealousy are minimized.

Biology whispers within us all, but it doesn't shout. Unlike other animals, we make conscious choices. Each person--gay, straight, in between, and off to the side--is an evolutionary experiment, so go ahead and experiment.

Just don't forget that your mother is watching.

Judith Eve Lipton, M.D. and David P. Barash, Ph.D. are the authors of The Myth of Monogamy: Fidelity and Infidelity in Animals and People (Freeman, 2001). They are heterosexuals.