For years now, marriage laws in every country have dictated that only two single adults of opposite sexes may legally marry. Great strides have been made in striking down the "opposite sex" requirement, and few people agree that bigamy, being married to someone while still legally wed to someone else, should be allowed. The only issue that remains in question, then, is the number.

Why should it be two? Why can't three people, for example, be allowed to marry each other if they so choose?

The answer lies in the inherent contradiction presented in this argument. In order to justify a three way marriage, all parties would have to be in love with each other. Unlike bad pornography, a willingness to perform sex acts with two other people is not the basis for marriage. For a marriage to work between three people you would necessarily have to start with the requirement that this is a completely mutual love relationship. This is an unlikely scenario for myriad reasons, not the least of which is that it is hard enough to find one person who represents all your values, let alone two people, who would also have to be in love with you and each other.

But the main reason that this kind of relationship can't work also illustrates why other marriages, those between two people, do work. A marriage of three violates two of the most basic tenets of successful marriage, two qualities which we embrace and understand on a certain level yet hardly ever articulate: primacy and privacy.

The concept of primacy is perhaps the most important ingredient in any successful marriage. It is the very reason we enter into monogamous, lifelong marriage in the first place; when we envision love, we think of being the most important person in another person's life, and vice-versa. This amounts to being primary in each other's life, of occupying a select and special place in the hierarchy of love and emotion. It is a position we don't come by easily; love and devotion on that scale require much of us. We must be the best person we can be, we must bring the highest qualities to our marriage, and to our partner's life, we must continually earn that position, through love, certainly, but also through the protection and growth of our own inner character. Primacy means not only being that important to someone, but deserving to be so as well.

Primacy is the only thing that allows us to marry someone who has been married before, or to take a new husband or wife if we ourselves have been married before. Primacy establishes that although someone else was important to us, they are no longer so and consequently we accept the primacy of the new person in our lives. Without it, we would view marriage as though it were a pinata; only good once. Many of the problems couples face revolve around the issue of ex-lovers and future rivals, commonly referred to as jealousy but in reality, an indication that primacy is threatened. It is crucial in a relationship as intimate and emotional as marriage that our partner's trust in us not be rewarded with questions as to whether that trust is well placed, or their willingness to be emotionally vulnerable to us rewarded with doubts about whether they are primary in our affections or not.

Couples thrive when they continually refresh the notion of each other's primacy. Genuine jealousy is kept at bay when each partner knows their value to the other, when they share experiences, thoughts and feelings on an intimate level that no other relationship can equal, when there is no other relationship, past, present or future, that they must compete against.

Privacy plays a role in this as well. True intimacy, true marriage, is only possible when lovers are able to inhabit the world alone together, even if they can't be alone all the time. Couples must have a quiet corner for themselves, doors to close out their family and friends, time to experience only each other. Even in early adolescence, when we first find ourselves attracted to the opposite sex, we want nothing more than to be alone with them. Lovers don't even want the company of another person let alone a sexual relationship with one. It's little wonder that movie theaters are the staple of dating couples everywhere: it's dark and private - you can forget the world exists if you like, at least until the movie's over.

This desire for privacy among lovers doesn't ever go away, no matter how many years spent together or how many opportunities for privacy we've had. It remains the foundation of our sexual relationships. A couple may hold hands in public, may even kiss, but they only make love when they are alone. It isn't about shame, as some people think. It is about selectivity. Were we to share all of our deepest thoughts and feelings with anyone and everyone, were we to treat our sexual union with someone as though it were public domain, we would hold nothing sacred, and no amount of intimacy with any one person would bring us any joy.

Threeway marriages would make primacy and privacy impossible. By definition, you cannot have two primaries in life, nor can you share that role with anyone else. Neither can you be fully private with more than one person, especially if your relationship with them is sexual. The lack of these two essential qualities erodes many relationships, and destroys the intimacy necessary for marriage.

Human beings, for as much as some people claim otherwise, are monogamous by nature and by choice, and cannot be deeply intimate with more than one person at once. The rewards of primacy and privacy are too great for us to ignore the simple joy of being in mutual love with one amazing person. Anyone who wants to pursue other kinds of relationships does so at the risk of never experiencing profound love; there will always be a division of affection, an unbalancing of loyalties, a sense of distancing yourself from true intimacy by replacing quality with quantity. Any tales that do emerge of threeway sexual relationships always end the same way; two members of the couple become closer than either are to the third, and that person is abandoned in favor of a more exclusive, private relationship the new couple wants to pursue. There is no reason to believe the same wouldn't happen in a threeway marriage.

Monogamous marriage is the ultimate in romantic and sexual fulfillment; of all the inequities and exclusions of modern marriage laws throughout the world, the one thing they've all got right is the idea that marriage, in its best and only true form, means two.