Marriage as a 'social institution'

Of course, people reproduce in both senses outside marriage as they have throughout history. The question here though is why the state should take an interest in marriage itself. Marriage formalises what can occur outside marriage, gives it the imprimatur of the state because reproduction is judged vital to social stability and viability.

So what does marriage do that reproduction outside of marriage doesn't do? This has two aspects which match the two aspects of reproduction. Firstly, marriage law seeks to reduce the risk of genetic abnormality among children produced by the union. In Australia, it forbids marriage with three groups: siblings, "ancestors" (parents or grandparents etc) and "descendants" (children or grandchildren etc). Some countries also outlaw marriage to first cousins, uncles and aunts.

Secondly marriage law seeks to promote permanence. This permanence is not about love so special it lasts "till death do us part". The legal structure of marriage along with the vows made during the wedding ceremony are intended to compensate for the failures of love, an emotion as fickle as any other. Rather the aim is to provide a secure, safe and dependable environment in which to raise children. And that's because a secure, safe and dependable environment is actually what is required to raise children.

So, this is what marriage as a social institution is about – securing the future of society. It is the institution endorsed and supported by the state specifically for reproduction. Whatever else it might be, same-sex love does not and, indeed, cannot provide that link to the future of society. Same-sex love may be as sincere and authentic as that of heterosexual couples (or not as the case may be) but it is entirely private simply because it lacks the potential for reproduction. As an emotion, heterosexual love is no different but it has the potential for reproduction and thus it is public in a way homosexual love can never be. This is why the state endorses marriage. Love is love, certainly – but the slogan is simply irrelevant.