(To read part one of this series, go here. For part two, go here.)

Too many Christians and conservatives are either surrendering or retreating from the public argument over marriage. But we have hardly begun to fight! Precious few of us know how to make the case. We get impatient when it comes to learning multi-step arguments. Instead we may paraphrase a Bible verse or two, start every sentence with, “As a Christian I believe …” or crack a joke, “God made Adam and Eve, not Adam and Steve.”

This is no way to defend marriage reality. Not only does it not convince fence-sitters. It confirms the accusation of gay activists that we want to impose sectarian ideas on society. We can, and should, do better. We need to be ready and able to mount thoughtful, public arguments in defense of marriage, arguments that will resonate with reasonable people regardless of whether they recognize the Bible as authoritative.

And it can’t be just a few of us doing it. There are too many minds to change. If marriage is a public good and a public reality, then all of us, not just a few public intellectuals, must mount solid, public arguments in its defense. If we can’t take the time and effort to properly articulate such an argument, why should we expect our post-Christian culture to learn and embrace our position?

Our situation is much like 1973 after Roe v. Wade was handed down. Many Christians were either tongue-tied or confused on the question of abortion, and many mainline (and dying) denominations came out strongly in favor of legal abortion. But within a few years, faithful Christians and fellow travelers shook off their brain-fog, gathered their strength and launched a counter-offensive. The culture of death has not yet been defeated, but we have made inroads and know how to frame the debate. The same must happen with marriage, and the sooner the better. We have to think. Only when we understand and can frame the debate properly can we hope to retake lost territory.

We can start by getting three things straight.

1. Keep Separate Issues Separate

Don’t conflate separate questions. The debate over marriage is not about the origin of same-sex attraction. It’s not about what consenting adults should be allowed to do in private. It’s not about “homophobia.” It’s not about what you “believe as a Christian.” It’s not even about the immorality of homosexual acts, a related issue that Robert Reilly tackles in an important recent book.

It’s not about restricting certain kinds of marriage. It is always and everywhere about the nature of marriage and its role as a public institution: Is marriage a thing, and if so, what is it?

2. Recognize that Marriage is a Comprehensive Union

Strip away the diversity of cultural traditions — giving dowries, exchanging rings and vows in a church, throwing parties and bouquets — and what remains? What is marriage? Amidst the diversity across cultures, what is striking are the common features, recognized even in ancient cultures that were broadly accepting of homosexual practice. In an article and a later book, Sherif Girgis, Robert George and Ryan Anderson ably boiled down the basic ingredients of marriage:

Marriage involves: first, a comprehensive union of spouses [husbands and wives]; second, a special link to children; and third, norms of permanence, monogamy and exclusivity.

Notice the word “norm” — ideal. The norm is the same even if some marriages fail to fully achieve it. A proper end of the marital act is children, even if a child doesn’t result from every conjugal act, just as a proper end of playing football — to take a trivial example — is to score touchdowns, even if in some football games, nobody scores a touchdown.

Cultures have varied on the permanence of marriage. Cultures also have varied in their openness to polygamy, but even in polygamous societies, the marriages have not involved several men married to several women, all of them mixing and matching in various and shifting male-female pairings. The women in a normal polygamous marriage are all married to one man, and the norm of the one man/one woman conjugal union is typically preserved even in these cases.

We have good historical and religious reasons to legally enshrine monogamy over against polygamy, of course. (Even pagan Rome recognized this.) The point here is that even in polygamous societies, there remained the ideal and the norm of the conjugal pairing of man/woman.

The word “marriage” refers to a relationship different from all others. We relate to our co-workers because of our jobs. We relate to our neighbors because of where we live. We relate to our friends because we have common interests. In marriage, a husband and wife unite comprehensively, with their whole beings. We are spiritual and bodily beings. Any union that is comprehensive — all-encompassing — must include a union of bodies. But bodies can come together in all sorts of ways — dancing, shaking hands, wrestling, playing football and cramming into a crowded subway car. The connection of bodies that is a true marital union will fulfill a vital purpose that could not be fulfilled otherwise.

Think of the biology involved. Each of our organs has a biological purpose. The purpose of the heart is to pump blood, of eyes, to see, of lungs, to draw in air and capture oxygen to supply the body. All of these organs also have a common purpose: they work together to allow our body to live and thrive. Moreover, these organs are complete: they don’t need another human being to fulfill their function.

Each of us, however, has one biological function that by ourselves we cannot complete: sexual reproduction. In women, this includes both the sexual organs and the lactation systems in breasts. (The lactation system requires another human being, a baby, to fulfill its proper end.) Marriage is the institution that frames this natural reality. It intrinsically involves a man and a woman, and is intrinsically related to the bearing of children. That purpose can only be fulfilled by a specific kind of union with another human being of the opposite sex.

Male and female are “made to fit,” and until recently, probably no one anywhere thought to deny this. To reproduce naturally, to produce a new human being, a male and female must unite their bodies in the sexual act. In this one way, we are naturally incomplete as individuals and organisms. “Marriage,” as one scholar has written, “proposes a reconciliation of the most fundamental natural difference among human beings — sex.”

Of course, we’re not merely physical bodies. Our minds, emotions and souls work in harmony with our sexuality. Sex within marriage is a good thing in itself, even when conception doesn’t result, but the act still naturally tends toward reproduction — toward children. It has the awesome power to produce new human beings. To pretend otherwise, to isolate sex from childbearing, is to court disaster.

The sex act (rather than conception or childbirth) is the consummation — the seal — of marriage. Marriage protects, reflects and reinforces this powerful, complementary, reproductive part of our natures. No relationship between two men or two women can qualify as marriage because this conjugal act is impossible for them.

An infertile man and woman can still marry, since, as the National Review editors wrote in 2010, it is “mating that gives marriage its orientation toward children. An infertile couple can mate even if they cannot procreate. Two men or two women literally cannot mate.” As the editors went on to explain: “A child fulfills the marital relationship by revealing what” that marital relationship is, “a complete union, including a biological union.”

Same-sex unions can’t bring together complementary organs and body systems that are designed to procreate.

Marriage, then, is a comprehensive union of body, mind, emotion and soul, a proper end of which is children. This is why the state and society as a whole have a stake in recognizing it, and setting it off from every other institution.

3. Don’t Abandon Complementarity

Men and women, certainly in our bodies but also in other, less tangible ways, complement each other. One reason Christians may have been slow to make this argument is that many Christian denominations, under the influence of feminism, have abandoned the idea of complementarity, and now treat the idea of innate, God-given differences between men and women with suspicion. This is a fatal mistake. Those Christian bodies that fail to defend male-female complementarity in the next few decades will meet the same fate as those denominations that threw in their lot with Planned Parenthood. They will be assimilated, cease to be meaningfully Christian, and decline. The counteroffensive will fall to those Christian bodies that have resisted the cultural currents.

In short, given the nature of marriage, it doesn’t make any sense to refer to a relationship between individuals of the same sex, no matter how intimate, as “marriage.” The right response to talk about men “marrying” each other is: “You keep using that word. I do not think it means what you think it means.”

Jay Richards is Executive Editor of The Stream. Follow him on Twitter.