Faithful in our fashion.

DICTIONARIES DEFINE MONOGAMY AS "THE CUSTOM OF BEING married to only one person at a time." After a time, when two people no longer wish to be together, it's natural for them to move on to other partners. Of course, no one enters into a relationship with thoughts of moving on.

Gay men jokingly refer to serial monogamy as "love 'em and leave 'em" or, when carried to excess, "jump 'em, hump 'em, and dump 'em." Often gay men remain best friends with past lovers, who become extended members of gay families. Lesbians practice serial monogamy too, perhaps with more grief associated with the breakup and moving on.

Straight couples, under the marital bonds imposed by religion, are supposed to shun serial monogamy, which for them is divorce and remarriage. But with heterosexuals divorcing and remarrying at a greater rate than ever, many are serial monogamists. Ronald Reagan is a serial monogamist. So is Bob Dole. And Newt Gingrich.

I've devoted the past decade of my professional life to writing about the origins of things, from comets to condoms. Recently I published Sexy Origins and Intimate Things: The Rites and Rituals of Straights, Gays, Bi's, Drags, Trans, Virgins, and Others. I know something about monogamy historically--and personally: I've lived with the same man for 25 years.

In the earlier decades of this century, anthropologists thought monogamy was the greatest stage of development. But studies show that primitive cultures--invariably run by men--favored polygamy (more than one wife at a time). Monogamy arose because economic hard times made having many wives prohibitively expensive or the,ratio of women to men was unfavorable and because many religions equated monogamy with sexual fidelity. It was religion that demanded monogamy between lovers be a lifelong bond from which the only escape was adultery.

But in nature, as mirrored in gay male life, monogamy does not necessarily imply fidelity. A female bird, monogamous for life, will secretly offer sex to another male in exchange for nesting material superior to the twigs and grasses brought home by her mate. They remain a family but benefit from her contacts. Many animals, monogamously paired for life, "cheat" in some advantageous way.

Gay men understand this. Two gay men, after years of fidelity, may open up their relationship to occasional affairs yet regard themselves as a couple for life. My lover and I are such a pair. For our gay friends in relationships of 15 years or more, monogamy without fidelity is the rule, not the exception.

The benefits are threefold: The long-term companionship of a best friend, the carnal pleasure of a new affair, and an honesty about the behavior that renders cheating and deception unnecessary. Boyfriends from affairs are not discarded but integrated into the extended gay family.

Open monogamous relationships among lesbians do not fare so well. They are even rarer among heterosexuals. Perhaps it's because women, in their evolutionary role as mothers, equate commitment with fidelity. Men, gay and straight, whose evolutionary imperative is to spread their seed, understand that I the horny impulse for a quickie with a stranger need not diminish the love one feels for a mate. Many straight men in monogamous relationships dream of multiple infidelities. Others realize the dream but are plagued by guilt and fear of being caught. Many gay men live the dream openly and honestly.

Some anthropologists believe society is headed toward an era in which lifelong monogamy without fidelity may be the norm. There's something to be said for a partnership that avoids the heartbreak of separation, the acrimony of divorce, and the unpredictability of a second marriage--which might last no longer than the first. In evolutionary terms gay men may be pointing the way.

Panati, a physicist and former science editor at Newsweek, lives on Long Island, N.Y. Sexy Origins and Intimate Things is his 13th book.