There is prudence in not trying to fight along the lines of battle as one wishes they were drawn or thinks they should be drawn, but rather as they are in fact drawn. Otherwise, one stands to lose the battle, as one’s strategy ceases to be informed by the actual state of things.
This goes for the intellectual and cultural battle over the nature of marriage and the preservation of its integrity. While we may try to fight this battle from the standpoint of the natural teleology of marriage originally understood as the begetting and raising of children, for all practical purposes it seems that in many important ways ground here has for now been lost. For a whole slew of reasons – i.e. an assumed unsound understanding of the human body and its place in the existence of the human being, deep mistakes about the end of man, a lack of a proper and deep understanding of nature in light of formal and final causes active intrinsically within it, a pervasive forgetting of moral good and evil as being the rational and free ordering or disordering of human life and things in face of the finality of man and nature – people generally are practically unable to see that the intrinsic ordering of sexuality is first and foremost an ordering to offspring, and that any action contrary to this ordering is bad. We may desperately wish to fight the battle on this hill, as absolutely considered it is the best and most reasonable hill on which to fight it; but for all practical purposes we have lost the hill.
Rather, marriage is now seen as the enshrinement and (something like) a making firm of the love of two human beings as pure persons or pure subjects and not as human beings considered in their totality, and a celebration of this enshrinement and making firm. In light of this, there seem to be two purposes why many still bother with marriage, given the sort of sexual license that is now taken as normal and good. As many have pointed out before, and as seems clear enough, one reason is that marriage serves as a kind of validation of what the parties take (or wish to be) a true expression of their love by means of their sexuality. (Even if it is denied, as Roger Scruton has pointed out in his book Sexual Desire, this is already an admission that the body in a certain sense is the one loved, as the lover takes himself to reach a union, or seek a union, with the beloved through bodily, sexual engagement, and vice versa; but never mind this for now.) Second, marriage is taken by most concerned to serve as a means of ensuring that, through making it firm and enshrining it, the love those parties getting married have for each other is protected, preserved.
I would aver that the latter reason is probably operative in the minds of at least some of those of homosexual inclination as well as people who find themselves blessed with a properly ordered sexuality. It does not seem prima facie impossible that two homosexually inclined persons may have a love for each other, something of a willing of each other’s good, even if they are mistaken about what counts as an ordered expression of this love. Looking from the outside, and in a way that is free from all that colors and shapes a sexuality bent toward homosexual activity and the psychology and emotions of the one whose sexuality it is, this does not seem so: if one considers with a clear mind what sodomy proper involves, it is disgusting; and mutual masturbation, at best, is perversely childish. But there is much that can happen to one that prevents him from seeing what is obvious to others, and what is obvious in itself; people can act toward another at junctures in his life when he is especially vulnerable in such a way that from that he takes what are in fact perversions to be that through which love is expressed and experienced. In any event, Plato seems to have thought that there can be love between homosexually inclined persons even when its expression is sexual (although even he admitted that this should be transcended, finally – see his Phaedrus); and the fact that one can make errors in the specification of the principles of practical reason in such a way as not to be fully culpable, and considering the nature of sexuality as in part an ordering to union with another on account of the actual or potential, or merely apparent, good found there, seems to suggest that Plato had a point, and that what is stated above can obtain.
So, let’s take those homosexually inclined who wish to be married with another such as they at their word; let’s assume that there is indeed love there that they wish to see protected, preserved. Given that marriage serves at the same time as a validation of a sexual expression of the love of the parties getting married as a legitimate expression of this love, we can ask: Is homosexual activity in harmony with and supportive of true love, or does it hinder and damage, even destroy, true love? That is, as whatever else it is love is a willing of the good of the beloved, is homosexual activity assist or hinder the realization of the beloved’s good? And if homosexual activity is opposed to love, the good of the one loved, can such a relationship be enshrined and made firm if one really loves the one whom he would wish to marry? All accept that love is in fact love, and that it is a good worth preserving. Let’s take the battle there.