Neither perspective is very illuminating when one wishes to debate social policy. Instead I think that one has to ask the question: Who 'owns' marriage? Whose institution is it?
If marriage is a social custom of the religious heterosexual minority, then it is quite correct that allowing gay people to marry is offensive to religious faith, clearly inappropriate in terms of any historical context, and an abuse of state power. On the other hand, if marriage is really this type of faith-based and sexuality-based institution is also follows, incontrovertibly, that married people should have absolutely no legal advantage or preferred social standing compared to unmarried people. Because of the relative majority of married people in the older (and hence more powerful) layers of society, differential treatment of married vs. unmarried people, for example in tax, employment, children, or benefits, should be clearly denounced and struck down as a form of discrimination. In public life, marriage should be irrelevant.
If marriage is not a private affair of religious and heterosexual people, but is an institution of the state, then it should follow, without even any discussion, that no particular faith or sexuality entitles some citizens to claim that marriage should be modelled to suit them at the exclusion of others. Rather, if the state admits an institution to recognize and give benefits to people's partners, that institution should be as egalitarian, open, and culturally neutral as possible. As with any civil institution, It should not matter whether the partners are women, men, transgendered, or any combination thereof. In terms of number of partners, either the state should recognize both traditional polygamies and modern polyamories, or it should insist that each person may nominate one partner and that is it.
Personally I would take the first route, that of disestablishing marriage. To me, the modernization of marriage is a bit like reforming slavery until it is non-coercive, non-racist, and includes fair pay, worker's rights, a healthy and respectful working environment, and union representation. Certainly a valid path to modernity, but personally I'd rather abolish a concept with such rotten historical roots. Similarly, I would prefer for marriage to be stripped of any legal or de-facto standing until it is no more than a romantic, offensive anachronism, like chivalry or the church. I realise that this is an extreme point of view. Those who are already married, or who wish to be married, would complain that their investment in marriage is being taken away from them. But then if marriage is to be anything more than an optional anachronism, it should be reformed into a secular, multicoloured, and bland 21st century institution.
Western democracies are in fact taking a stealth middle route. Without challenging the symbolic status of marriage, they have weakened its substance from an institution where the man possessed the woman as a sexual and reproductive slave to one where marriage is merely a declaration of co-dependency and alternative tax treatment. This reform (brought gradually through the abolition of the dowry, abolition of adultery as an offence, prosecution of assault and rape against one's wife, equal economic relations, straightforward divorce, equal parenting rights, statutory maternity leave and child support benefits, full rights to unmarried parents and their children, etc.) was a vast and sweeping change to the character of marriage. Crucially, it is no longer a union that regulates either sexual behaviour or reproduction. Relaxing the rules about the sex of permitted partners is, by comparison, a very modest change in substance. It is, of course, a serious challenge in symbolism..
As our societies evolve, our institutions become less biologically grounded and more focused our relations as minds. I see the admission of same-sex marriage as a beautifully allegorical example of this trend.