Why is the Catholic Church so insistent that the institution of marriage can only occur between one man and one woman? Because the Church proclaims the truth about the greatness of human beings. And it’s because of this truth of our human greatness that the Church affirms that nothing less than the lifelong union of a man and a woman — freely giving themselves to each other in mutual love — is capable of being an authentic marriage.

This reality isn’t a negative proscription about who should or shouldn’t be allowed to marry. It’s a radically positive expression of who we are, and of what we are invited to become in this most intimate of human relationships.

In its campaign to legally redefine marriage laws in the United States to include same-sex couples, the homosexual lobby has caricatured its opponents, including the Catholic Church, as narrow-minded bigots bent on perpetuating a cruel injustice by denying homosexuals an “equal right” to marry their same-sex partners.

Against this false but powerful caricature, it’s inadequate to highlight the negative behaviors that are strongly associated with the homosexual lifestyle, to cite the adverse consequences that must confront children raised within homosexual “families,” or to point out that until very recently no human society has entertained seriously the claim that marriage can involve persons of the same sex.

These facts certainly are important, but negative arguments don’t win hearts and minds. As Pope Benedict XVI said in an August 2006 interview with German-speaking media, “Christianity, Catholicism, isn’t a collection of prohibitions: It’s a positive option. … We’ve heard so much about what is not allowed that now it’s time to say: We have a positive idea to offer, that man and woman are made for each other, that the scale of sexuality, eros, agape, indicates the level of love, and it’s in this way that marriage develops, first of all, as a joyful and blessing-filled encounter between a man and a woman, and then the family, which guarantees continuity among generations and through which generations are reconciled to each other and even cultures can meet.”

So Catholic Americans have a duty to state their case positively too, when defending marriage and family against legislative and judicial attacks — attacks like the one just launched in Congress to repeal the federal Defense of Marriage Act, which we report about in this issue of the Register. That’s what Bishop Richard Malone of Portland, Maine, is doing, as he leads the fight in that state to place a “citizen’s veto” on the state ballot in November asking voters if they want to repeal a law passed this spring granting homosexual couples the “right” to marry.

“A same-sex union can never realize the unique and full potential that the marital relationship expresses,” Bishop Malone has insisted.

But while most people retain an intuitive understanding that true marriage is a heterosexual-only institution, it’s not always easy to know how to make that case in a positive and persuasive way.

Fortunately, there’s plenty of help at hand. The Catechism of the Catholic Church, numerous Vatican documents and especially Pope John Paul II’s rich theology of the body are all superb resources for articulating the truth about love and marriage.

The Catechism succinctly describes the two positive dimensions of authentic marriage: its “unitive” capacity to create an intimate communion of “one flesh” that joins the complementary male and female natures of husband and wife; and the “procreative” capacity of this intimate communion to generate new human life.

States the Catechism, “By transmitting human life to their descendants, man and woman as spouses and parents cooperate in a unique way in the Creator’s work” (No. 372).

And, as John Paul taught in his theology of the body, in a mysterious yet very tangible way, this loving union of husband and wife, and the new human life they can generate together, mirrors the communion of the three divine persons of the Holy Trinity.

To be matrimonial mirrors of God the Father, God the Son, and God the Holy Spirit — that’s truly a call to human greatness.

Also of much value in communicating the positive truth about marriage are the resources made available by the U.S. bishops at the “Marriage and Family” page of their website (USCCB.org/laity/marriage/index.shtml).

The ideal model for positive, effective communication is Jesus himself. Asked by a scribe to summarize the largely negative rules regulating human conduct that were communicated by God to Moses in the Ten Commandments, Jesus restated them as two positive, condensed commandments — to love God and to love one’s neighbor.

And when the pharisees sought to test Jesus by asking him about the validity of divorce under Jewish law, he seized the opportunity to deliver a bold affirmation of the indissoluble character of marriage.

Moses had only allowed divorce because of “the hardness of your hearts,” Jesus said in reply. But, he added, God’s intention is that men and women who are joined in marriage should never be separated.

In effect, Jesus was telling the pharisees, “As persons created in the image and likeness of my Father, you’re much, much bigger than this question of yours implies. Marriage isn’t about negative rules or damage control; it’s about God joining husbands and wives together for life in a joyful merging of their masculine and feminine identities.”

Jesus was saying, “Think ‘great,’ when it comes to marriage between one man and one woman. Don’t be so small-minded and hard-hearted.”

That’s the same Christian message the Church continues to deliver today as it addresses the destructive bid to redefine marriage to include same-sex couples. It’s not that the Church is wedded to a view of marriage that’s too small and too narrow to accommodate the full range of human love. It’s that the Church knows that the dignity of the human person is too immense to be compromised through a pretense that this counterfeit redefinition is equivalent to the unique marital union that exists between a man and a woman, joined together in love.