Marriages traditionally were always intended to further the welfare and interests of society, not of individuals. The endorsement of the male-female union was merely to encourage a healthy, burgeoning growth of the general populace through the triadic relationship amongst the provider, the homemaker and the offspring. Innumerable legal benefits are provided to married couples for this reason. When a person has a family and children, that person is considered by society as a prima facie stable and respectable individual.
Traditionally marriages were more about fulfilling a certain familial/societal obligation as a member thereof, than adhering to personal whim or attractions to another. Marriage was never about endorsing love per se, nor even about endorsing love between a man and a woman. Love is a generic catchphrase - a fairly RECENT, Westernized concept/creation of mythical illusion - to create the impression that marriages are the byproduct of love, and to make marriage more attractive to the modern generations.
Society needs procreation to survive; same-sex unions cannot provide it and that is the main reason they were discouraged by society, deemed abnormal by the community, and condemned by most forms of religion. As the communal values of morality, diversity and tolerance shift and evolve, same-sex marriages will (has) eventually come to be tolerated, accepted, standardized, and ultimately (if not already) commercialized.
The idea that marriage is (should be) a byproduct of love is, in my opinion, wrong and repugnant (it'll require a separate thread to explain my reasons) to my sensibilities. However, that appears to be the current social climate - as long as two consenting, reasonable adults love one another, their sexual preference should not preclude them from receiving the communal, legal, and personal benefits that marriage arguably grants. I happen to agree with the conclusion, but for a different reason which I cannot get into here.
Parent-offspring unions have a whole separate problem, however. I'm sure truthteller was just rhetoricizing about it, but surely he understands that a parent and child (even at over 21 years of age) cannot by definition be on the same wavelength in terms of mutual power and respect. Add to it the practical problems of how their children should be defined (i.e. mother's son or father's grandson?) and the possibility of genetically-induced difficulties, and this is a no-go. Animal-man unions are not acceptable by the current social climate because such union is not indubitably consensual, along with about a million other reasons that are too obvious to discuss.
My stand on the issue of same-sex marriage is that it should not be an issue, because marriage as a concept is defunct and obsolete. The attempted coexistence of marriage and love is, for the most part, a failure, and whether it be between different sexes or the same sex, acceptance of love between two consenting adults does not entail acceptance of marriage as a standard to gauge that love.
[minor edit for clarification]
