The Rev. Lindsay King of Toronto's Willowdale United Church once announced he would unite two mynah birds in holy wedlock.

Rajah and his prospective bride, Rani, could easily be coached to caw the words “I do.” And even though the vocalization of their vows would not come from their hearts, Pastor King thought that the ceremony would be refreshing and “emphasize the concept of love”.

The church wedding, however, did not take place.

A barrage of letters and telephone calls from irate members of his congregation persuaded the Protestant pastor to cancel the connubial service for his feathered friends. People insisted that marriage is for a man and a woman.

Yet, on Jan. 14, in the same city of Toronto, the Rev. Brent Hawkes celebrated the “marital union” of two men and two women. Because his church, the Metropolitan Community Church of Toronto (headquartered in Hollywood), is homosexual, there was relatively little protest when the banns for these unions were announced.

Nonetheless, protests have been intense and continuous. Ontario's consumer minister, Bob Runciman, has promised not to register the “marriages.” His promise, as expected, has fueled the controversy and precipitated the predictable charges of “discrimination,” “homophobia” and “imposing right-wing values.” An editorial in the Toronto Star claimed that the church's plan is “ground-breaking” and compares it with the suffragette movement.

Is there any solid basis on which one can argue that sex — that is, the complementarity of male and female — plays an indispensable role in forming a true marriage?

Looking at the issue from a historical perspective, we see that homosexual acts were accepted in ancient times.

It was the Jewish Torah that found such acts to be an “abomination.” And it was Judaism alone, 3,000 years ago, which denounced homosexual practices, while insisting that all sexual activity be channeled into marriage, a two-in-one-flesh union between a man and a woman. In other words, it was marriage as a faithful and intimate union between a man and a woman that was truly groundbreaking.

Moreover, this form of marriage, together with its resulting family, has been a source of great strength and stability for Western civilization. Jewish scholar Dennis Prager has remarked that “[t]he acceptance of homosexuality as the equal of heterosexual love signifies the decline of Western civilization as surely as the rejection of homosexuality and other non-marital sex made the creation of this civilization possible.”

It was Christian marriage that was groundbreaking.

Marriage is more than a loving relationship between two people. Love is not restricted by gender, age, race or social status. Love is universal. But marriage is subject to restrictions, precisely because of its nature as a two-inone-flesh union.

Whereas each person is equally lovable by another, it cannot be said that each person is equally marriageable for that other. The essence of marriage is not equality, but two human beings who are properly proportioned to each other in accordance with the nature of marriage. As an institution, marriage transcends individuality, personal will and political fiat.

The science of immunology provides important insights that help us to understand why a same-sex relationship cannot exemplify a two-in-one-flesh intimacy.

An individual's immune system contains 100 billion immunological receptors. Each of these receptors has the remarkable capacity to distinguish the self from the non-self. In this way, the immune system protects the self from the intrusion of anything that is foreign and potentially dangerous to it. The immune system fights off diseases, but is also set up to reject organ transplants, since it recognizes the organ transplant as something alien to it. From a purely immunological standpoint, we are all alien to each other.

During heterosexual intercourse, something very special takes place, immunologically speaking. The male semen contains a mild immunosuppressant which, when released into the woman's body, alters her immune system just enough so that the man's sperm is received as part of herself.

This acceptance and mutual identification on an immunological level is also necessary to prevent the woman's body from rejecting an ensuing embryo as if it were a foreign substance. In other words, through sexual union, a man and a woman truly achieve a two-in-one-flesh unity. And the child that is formed as a result of their union is the fruit of that union. An intimate union of this kind is not possible between same-sex couples.

The nature of heterosexual intercourse is uniquely ordered so that the male and female participants are not only profoundly united to each other — body and soul — but also involve themselves in an act through which they can confer upon each other the blessing of motherhood and fatherhood.

This is why marriage, in the truest sense, requires a man and a woman. People who share the same sex can love each other, be friends, or remain lifelong allies. But they cannot be married to each other according to a two-in-one-flesh intimacy that is intrinsically ordered to parenthood. This is not a political restriction, but one that is natural. And we cannot de-legislate nature by a political act.

The essence of marriage must be honored. It is not politicizable. It is what it is, a two-in-one flesh unity between a man and a woman. As Rev. King's congregation reminded him, anything less is strictly for the birds.

Donald DeMarco is a professor of philosophy at St. Jerome's University in Waterloo, Ontario.