Linda Casper and Stacey Shelton appear to be just like any other plaid-shirted, plain-spoken, well-scrubbed Midwestern couple. Linda's a buoyant, gregarious redhead with a thick helmet of hair and an expansive smile. Stacey's boyishly handsome, bespectacled and taciturn, with a shy smile -- the kind of guy women want to bake brownies for. She sells log homes; he's a mechanical designer who retools muscle cars in his spare time. They're both volunteer firefighters in their hometown of Exceland, Wisc., which boasts a grocery store, a Chevy dealership and two bars. They have two daughters, ages 5 and 10 months, and about their marriage, Linda says, "I love my husband, and would never leave him."

Yet once or twice a month, Linda, 25, and Stacey, 28, drive three hours from their rural outpost to the Twin Cities, where Linda has sex with her lover, Steve Adams, while Stacey and Steve's wife, Aleta -- who's involved with a man named Mark -- go to the mall. Linda, who's bisexual, sometimes also goes to bed with her friend Mary Anne. Also, she's shopping around for another female lover. Lest you think that Stacey's a world-class cuckold, consider that he took a lover soon after his marriage, he once did the nasty with their former roommate, Sarah, while Linda was just a few feet away, "slaving outside" and he's currently in the market for another woman for both he and Linda to bed.

Know what? It's all OK.

Linda and Stacey practice polyamory, the policy of loving more than one person at a time, and they're part of a growing number of Americans quietly riffing new, post-monogamous arrangements on what they consider a tired two-by-two tune.

They're not swingers! Or patriarchal, oppressive junior Mormons. Nor are they trying to deface the Ozzie-and-Harriet domestic blueprint stained by pandemic divorce and infidelity. Instead, their mantras are responsibility and commitment.

For example, Linda and Stacey have asked Steve and Aleta to live with them. Linda says, "I don't think I could be monogamous. I tried it, and it wasn't me. I think polyamory is very healthy; it promotes more honesty and communication."

Polyamory is an umbrella term for such variations as group marriage (for a man, say, it would be you, your wife and your blond divorcie neighbor living together and sharing resources equally); polygyny (the Mormon model: same cast, only you're the sole provider); "intentional community" (you, your wife, the blond, the blond's other lovers, all your friends and the pizza delivery guy living in a big house in Boulder); and "intimate network" (a kind of off-site commune with primary, secondary and tertiary levels of intimacy in a kind of erotic farm system). Then you have your expanded families, open marriages and line marriages, the latter concept far too esoteric for such a venue as this.

It all gets very confusing, even to the participants. When asked how many lovers she currently had, Deborah Anapol -- author of the book "Polyamory: The New Love Without Limits" -- replied, "I haven't kept count lately, but to round it off, I'll say 12."

Since polyamorists themselves sometimes can't tell their players without a score card, trying to estimate their national numbers gets even dicier. Although no studies have been done, experts such as Ryam Nearing, who claims to have started the polyamory movement in the mid-1980s, estimate the number of American "polys" to be from 8 to 10 percent. Anapol claims her Web site gets thousands of hits daily, while her book is entering its fourth printing.

Brett Hill, Nearing's partner and editor of their magazine, Loving More, argues that "poly is the next wave of human relationships. First there was blacks, women, gays, now us. When we started 15 years ago, there were no support groups; now there are at least 50, in almost every major city in the country."

Polyamorists range in age from 20 to 70, with most in their 30s, 40s and 50s; most are white, and they range professionally from laborers to CEOs to academics. They meet through personal ads ("Married ex bi-curious straight-again crone seeking 2 M for MFM triad"); at regional workshops run by Nearing and Anapol; at local PWP's (Poly Want a Potluck); in virtual polyland, joining chat rooms such as the "lovelists" at Nearing's Loving More site and live discussion groups, where the topics may include: how to ask somebody for a date when you're already married to six other people, how to prevent jealousy when your "primary" spends every other night with his "secondary" (do the math) and how to explain to neighbors that the strange man who just moved in with you is your "wife's uncle from out of town."

One chilly Sunday afternoon in Greenwich Village, a dozen and a half people shuffle into a back room of the Cafe Figaro. They're mostly in their 40s and 50s -- earth mothers and fathers in jeans, flannel shirts, bandannas and love beads. It could be a board meeting of Ben & Jerry's, rather than the monthly conclave of the Tri-State Poly discussion group.

Joel Spector, a short, bearded man with a dry wit and leprechaun-ish mien, founded the group in 1994. He's the only one present who lets me use his real name. Joel's married to Christina and involved with Tracey, a married woman from West Virginia whose female lover will soon move in with her and her husband. Marie, a young woman with a black skullcap-like hat and a minister in training, is "married" to Suzanne and involved with Ray. John says he's with several women and is a private investigator, a "freedom freak" and into S&M.

The topic is "What is polyamory?" and it inevitably leads to a critique of monogamy. A woman named Paula -- whose husband George is involved with a woman he met at a Loving More conference -- asserts that "monogamy is fueled by fear and low self-esteem." A late-arriving woman announces, "The way most monogamy is practiced -- with everybody cheating -- is just a dishonest version of polyamory."

In contrast, polys claim that the bedrock of their faith is "radical honesty," and the group discusses how far to take it. Bob, a white-haired, avuncular type, says, "My wife doesn't want to hear the details of my sex life." Others ask: Is she insecure? Is she bored? Bob doesn't know.

The polyphony continues, engaged but mellow -- until I'm asked to identify myself, and the group suddenly turns paranoid and threatens to turn me into fish food. Many want me to leave. Marie questions my journalistic ethics, while Spector (who invited me without telling the others) defends my presence. An uneasy truce is reached -- I have to put my notebook down -- and, afterward, I learn the source of their outrage: a fear of being outed.

They may have good reason, based on a recent court decision in Tennessee. Last December, MTV broadcast a documentary about polyamory and interviewed April and Shane Divilbiss and Chris Littrell, who lived together in a male-female-male triad. April Divilbiss had a 3-year-old child, who was not mentioned on the show. The day after the show aired, the child's paternal grandmother, Donna Olswing, showed a judge a tape of the show and had the child taken into state custody due to the mother's "immoral" lifestyle. The mother is contesting the decision, and Loving More -- with a feeling that the Divilbiss case may be the polyamorists' Stonewall -- has set up a defense fund.

Despite the risk, some polys court publicity in an attempt to promote their cause. Nearing has appeared on daytime talk shows such as Geraldo Rivera's and Sally Jesse Raphael's, only to be castigated. "Geraldo attacked me for being indecent even though he's had a kid out of wedlock and been divorced three times," she says. Raphael blamed polys for the AIDS epidemic.

In the face of such hysteria, most polys stay fairly closeted, which might be just as well; they have enough to do just keeping track. You see, polys devote a LOT of time to what Wolfe calls "processing drama," the often-exhaustive hashing out of relationship mechanics.

A poly family is like marriage squared; the more people involved, the more complex the issues. "Let's say you're in a triad," Nearing posits. "You might get along with persons A and B, but A isn't getting along with B. A might want to spend more time with you than B. B won't like that. All this brings disharmony to your relationship circle, and you have to discuss it." (Don't feel bad if you're confused; to help clarify matters, people such as Nearing have taken to diagramming poly formations that resemble John Madden's depiction of the nickel defense.)

Since secrecy is verboten, polys also huddle to establish ground rules on everything from who does the dishes to dating outside your marriage/group/sacred circle.

Black Eagle, a 57-year-old retail store manager in Austin, Texas, lives with two women, Silver Moon and Snow Bird (none of the three is Native American). "Before I date a woman," he says, "she has to meet Snow Bird and Silver Moon and get an OK."

Even if the newcomer survives that powwow, she may run up against a logistical logjam. Black Eagle speaks: "We have a schedule for sex. Tuesday I'm with Snow Bird, Wednesday with Silver Moon, Thursday, Snow Bird; Friday, Silver Moon. Saturday night is family night; we stay up late and watch a movie. Sunday's open, depending on who's horny. And Monday's my night off."

As you might expect, even poly safe sex protocol can become awfully byzantine. Spector admits, "Our condom contract about sex outside the group runs to six pages, and details what can be done with whom, when and under what circumstances. I'm not pleased with it."

If this internal red tape weren't enough, polys such as Lynn Garcia of Portland, Ore., must also feint their way across a monogamy-rigged U.S. legal system. "I'd already had a child with my legal husband, Roger. Then myself and my co-husbands, Roger and Van, decided to have a second child, and that Van would be the father. So for legality's sake, I divorced Roger and married Van."

Polys aren't protected by property laws, either, an obstacle Casper and Shelton circumvented by registering their family as a corporation (legal in Wisconsin). This allows them to protect their assets in the event of death.

All this -- societal ostracism, marathon group therapy, sex time sheets -- seems a high price to pay just to avoid the inevitable entropy of marriage. So why do they do it?

One reason is sex. While polys won't admit to having more than the rest of us, they're pretty sure it's better. Robert McGarey, a "relationship coach" from Austin, attributes the added heat to polys' deeper sense of emotional intimacy, and thinks the poly slogan should be "It's the love, stupid."

Linda Casper says poly affirms her inherent need for sexual freedom: "Why should I give up my marriage just because I want to have a relationship with a woman?" She adds that it spices up her marital sex: "After I'd been with Mary Anne, Stacey would ask about it, and it would get him excited."

Pat Mathis, an Oklahoma City chiropractor, says, "Two things drive me toward polyamory: One, I can't get enough sex with just one woman, and two, it's hard to find a woman who shares all of my interests," which include tantra, a Heinlein-inspired religion called the Church of All Worlds and ballroom dancing.

Polys also claim that their lifestyle offers a sense of extended family, it's guilt- and jealousy-free, and it relieves the pressure of having to satisfy all of your partner's needs. Anapol says, "If you're married and your wife's too busy for you, you could feel rejected. But if you're with 20 people and 10 are busy, there are 10 other people around who love you and who you can do things with, sexual or otherwise."

For many, poly's just doing what comes naturally. In Casper's case, it may even be in her genes; her parents had an open marriage, and as a child, she lived in a house with as many as five polyamorous adults.

Still, even its most devout adherents admit poly isn't for everybody. Spector warns: "If you're gonna do poly, you've got to have your relationship shit together or it's not going to work."

Ask Leanna Wolfe, who realized she wasn't prepared to cope when Don, her partner of six years, suddenly acquired another lover. While most American women would've thrown Don out or pulled the interloper's hair on Jerry Springer, Wolfe, ever the anthropologist, headed to Africa. "I went to study the Luo tribe in Kenya to find out how women share men in traditional cultures."

What Wolfe discovered was that even to experts, poly is a bitch. "The Africans had trouble with it, and they've been doing it for thousands of years. Wives told me they had a hard time with jealousy and would get into physical fights with each other."

This begs the question: Is poly the wave of the future or a human potential movement backwater?

Dr. Susan Vaughan, a New York psychoanalyst and author of "Viagra: The Guide to the Phenomenal Potency-Promoting Drug," says, "It sounds like a nice idea on the surface, but I think people have affairs or do swinging because they want secrecy and adventure. Besides, with poly, the jealousy and rivalry is exponentially multiplied."

Wolfe agrees that come the millennium, we won't all be "nesting" in our communes: "Polys aren't spearheading any movement in this country. They're just too wacky, and they're never going to go over in the heartland."

She may be right. Back in Wisconsin, Stacey Shelton has decided he doesn't want to live with Steve and Aleta Adams after all. And Aleta, at posting time, was breaking up with her lover, Mark. But Linda, ever optimistic, has contacted a poly couple in North Carolina with the hope that maybe this one will be the match made in heaven.