Such a union defies the essential meaning of marriage. Maybe this can be best understood when we speak of the “marriage of music and dance,” or “the marriage of ideas.” For by that we understand that there’s something different and yet compatible between them for the word ‘marriage’ to be meaningful. It is meaningless to speak of the marriage of “music and music,” or the marriage of “this idea and the same idea.”
‘Homosexual marriage’ is nonsensical to the core. Such a phrase is a contradiction in biology, sociology, philosophy, and theology. Not merely because there’s no offspring, but because there’s nothing from which to spring forth.
There is possibly a temptation to rebut by asking: “Don’t any two men (or two women) have enough differences between them so a meaningful ‘marriage’ could be in order? But the intrinsic point of same sex attraction is their being the SAME and not being different at all. The other differences of personality, IQ, talents and the like are irrelevant to a discussion of same SEX attraction, i.e. homosexual marriage. An appreciation of the other fine qualities should be a discussion on friendships. Which, last I knew, was still an option without sex.
One might also object by pointing to the powerful feelings homosexual couples can have for one another. Such feelings must have some basis in meaning. Surely two people of the same sex could bring their own uniqueness to one another and simply create a meaningful relationship worthy of the name ‘marriage’. But to argue that way is not to argue for the common ground of marriage, but for the unique exception of the individual to supersede marriage. It is closer to arguing for cohabitation or even adultery. It is the antithesis of marriage, the very violation of it. The degree to which our society can not see this historically obvious truth is the degree to which we are already lost in a meaningless marriage of existence . . . and existence.
God made Eve different from Adam, yet humanly the same. Had God made another humanoid out of another plot of soil, such a creature would have been a totally alien being. (Cat vs Dog so to speak instead of bone of my bone, flesh of my flesh.) Had God made an identical clone of Adam, such a creature would have been too much the same.
And BTW, marriage is not a completing of oneself. It is a completing of humanity. Individually every human being is complete and valued as is. Humanity, however, is incomplete without male and female. Marriage is the synthesis by which humanity gets beyond being a mere collection of individuals or even animals. Marriage is a giving of oneself and a giving back in such a way that the two are greater than before. (It is also a reflection of the image of God who is himself a combination of three differing persons who are yet of the same essence constituting God.) But if all parties are identical they are not greater when joined together, just more numerous.
Marriage is about two becoming one. Homosexuality is about one and one being two.