Hm. I am not so sure why people are so quick to assume that the ‘primary purpose’ (talk about a priori presumption) is reproduction. It is certainly a function of sexual behavior, but why need there be a primary one at all? Indeed, I think the Catholic Church gives equal time to the ‘unitive’ aspect of sexuality.
Among humans and our closest genetic relatives among the anthropoid apes, sex serves at least as much as a function of social bonding, alliance and affection, and that includes same-sex behavior, whether mild or mock-activity, occasional sex, or lifelong pairing.
There are many working models of the small social unit (ie the ‘household’ or family). If one would condemn any households that are not entirely stable and prohibit them, then well…we may all be consigned to loneliness, whether we are gay, straight, any race or religion or combination of these.
And as to gay couples ‘not needing’ the benefits of union? How could anyone say that a couple who has lived together for decades, through thick and thin, whether or not they were financially well-off, should be denied next-of-kin status, particularly when one of them is hospitalized or dying? It breaks my heart, and breaks the hearts of their families and friends when this happens. Can you even imagine how it must feel to be automatically disqualified from being considered family of someone you have loved and has loved you all of your lives, let alone being acknowledged as a true and stable home?