“Marriage equality” has become a buzz-phrase in the advocacy of same-sex marriage in the latest use of relativism to equate what can’t be equated. Equality as a word has been repeatedly used in various advocacies as if it was an argument that can’t be refuted. I mean who can be against equality? Equality is seen as a basic form of justice as any kind of unfairness we naturally respond to. The problem is you just can’t put anything on either side of the equals sign and make the resulting equation true.
Really though the problem with same-sex marriage is with equality itself – it is too equal in fact. The fundamental reality of marriage is not based on equality but that that men and women have complementary natures. It is no coincidence that the growing acceptance of same-sex marriage followed the feminist advocacy of what comes down to making men and women equal via contraception and abortion. Once you render one of the ends of sex null than same-sex sexual activity seems to be as equal as contraceptive opposite-sex activity.
The equality of men and women is based on their personhood. “God created man in his image; in the divine image he created him, male and female he created them” (Gen. 1:27). God command to “be fertile and multiply” (Gen 1.28) can only be met because of the complementarity of men and women. Same-sex marriage or same-sex relationships can never be fecund because men and men or women and women are too equal both physically and psychologically.
228. Connected with de facto unions is the particular problem concerning demands for the legal recognition of unions between homosexual persons, which is increasingly the topic of public debate. Only an anthropology corresponding to the full truth of the human person can give an appropriate response to this problem with its different aspects on both the societal and ecclesial levels. The light of such anthropology reveals “how incongruous is the demand to accord ‘marital’ status to unions between persons of the same sex. It is opposed, first of all, by the objective impossibility of making the partnership fruitful through the transmission of life according to the plan inscribed by God in the very structure of the human being. Another obstacle is the absence of the conditions for that interpersonal complementarity between male and female willed by the Creator at both the physical-biological and the eminently psychological levels. It is only in the union of two sexually different persons that the individual can achieve perfection in a synthesis of unity and mutual psychophysical completion”.
Homosexual persons are to be fully respected in their human dignity  and encouraged to follow God’s plan with particular attention in the exercise of chastity. This duty calling for respect does not justify the legitimization of behaviour that is not consistent with moral law, even less does it justify the recognition of a right to marriage between persons of the same sex and its being considered equivalent to the family.
“If, from the legal standpoint, marriage between a man and a woman were to be considered just one possible form of marriage, the concept of marriage would undergo a radical transformation, with grave detriment to the common good. By putting homosexual unions on a legal plane analogous to that of marriage and the family, the State acts arbitrarily and in contradiction with its duties”. (Compendium of the Social Doctrine of the Church)