In many respects, the debate about according legal recognition to homosexual relationships, be it through civil unions, relationship acts, or marriage licences, is an argument about justice.

It is unjust, some say, to limit marriage's legal benefits to one man and one woman committed to a life-long, exclusive relationship.

The idea of justice reflects our recognition that every human being, by virtue of being human, enjoys equal dignity and so ought to be treated in certain ways. But does equal dignity automatically translate into equal treatment?

The short answer is: not always. There is, for example, nothing discriminatory about declining to issue a gun permit to a child.

This does not mean that the child is not owed equal respect as a person. But the child may be legitimately denied a shooting permit because a child is different from an adult with regard to guns. Here, discrimination is not unjust.

When we apply the same logic to the question of extending marriage's legal benefits to homosexual couples, which is what civil unions or their equivalents do, we begin to see that denying such benefits is not unjust.

Those claiming that homosexuals ought to be allowed to enter civil unions argue that a homosexual couple's relationship can be equal to a man and woman's relationship.

They maintain that elements such as the duration of a relationship between two persons, its sexual character, its mutual dependence, and its exclusiveness may be as present in a homosexual couple as in a heterosexual relationship.

These features do not, however, constitute the basic reasons why marriage is accorded legal privileges.

They are, rather, the necessary preconditions for that aspect of marriage that distinguishes it from other restricted, dependent, sexual, and lasting relationships.

This element is the distinct function of procreating and raising children.

By legally recognising this unique marital characteristic, the law affirms that society's common good is best served by children being born and nurtured in a family where they can feel and understand the reason for their irreplaceable, biologically unique self: the love of their father and mother.

By definition, homosexual relationships are incapable of serving such an end.

The existence of legal provisions for adoption (the assuming of parental responsibility for a child by persons with no biological relationship to the child) does not undermine this argument, but reinforces it.

The law has not traditionally permitted adoption because two people just happen to want a child, in the same way that they just feel like purchasing a car.

Rather, adoption is permitted because it is considered objectively good for a child to have the fatherly and motherly love and security that can only be offered by a man and a woman committed to an exclusive relationship.

Some may observe that not all marriages result in children. But though the law cannot know if a marriage will result in children, the law reflects the fact that only one woman and one man can be the biological parents of a child and raise it in an atmosphere of mutually reinforcing motherly and fatherly love.

Moreover, the fact that the law confers unique privileges upon marriage does not mean that homosexual relationships are being treated unfairly.

When we think about it, we realise that most two-person relationships, such as two business partners or two friends, do not qualify for marriage's legal benefits, precisely because they lack marriage's unique procreative, child-rearing potential.

What distinguishes homosexual relationships from other two-person relationships is their very specific sexual character: sexual acts that can never be procreative and never express the two-in-one-flesh unity uniquely manifested by what we call, for good reason, marital acts.

The fact, however, that homosexual relationships have a non-procreative sexual dimension does not entitle homosexual couples to marriage's legal benefits.

Their sexual acts do not have the exceptional potential significance for society's common good (the creation and raising of children through committed relationships of fatherly and motherly love) of a man and a woman who make a unique commitment to each other that is consummated by sexual acts of a reproductive type.

Indeed, such legal recognition might actually constitute discrimination against all those exclusive, enduring, dependent relationships that also lack a non-procreative, indeed non-sexual dimension, such as an unmarried son caring for his invalid father.

The ongoing debate about civil unions is about many things, but let us dismiss the notion that opposing civil unions is unjust.

To the extent that civil unions or any legal equivalent diminish the law's recognition of marriage as a special relationship of unique benefit to society, they appear contrary to justice's demands and society's common good.

* Dr Samuel Gregg is director of research at the Acton Institute, Grand Rapids, Michigan, United States.