The argument against same-sex marriage is based on the nature of human persons as beings with a gender who have a purpose derived from that nature. That is to say, male human persons are meant for coupling with female human persons, even if their coupling does not result in procreation. This argument is not based on a human person's function, ability, or desire, which could each be inconsistent with how human beings ought to be by nature.
For example, a person who is comatose, insane, sightless, or sexually desirous of his neighbor's young child lacks something either physically, psychologically, or morally. But he remains a human person who by nature ought to be conscious, sane, seeing, and desiring well. In the same way a sterile, aged, or willingly childless person is still a gendered human person whose purpose for marital union (unless called to celibacy) can only be consummated by one-flesh communion with a spouse of the opposite gender even if he or she has contrary desires.
Desires, after all, can be immoral and sometimes harmful to a human person's good, such as desires to overeat, commit adultery, molest children, and engage in gay bashing.
Francis J. Beckwith and Gregory Koukl, “Relativism: Feet Firmly Planted in Mid-Air,” pg.123