However, same-sex marriage would be the most radical change in the history of western civilization in terms of a social institution. It would in essence render genders obsolete.
Gay marriage is a more radical change than election by popular vote, the creation of the corporation, ending slavery, the notion of personal liberty, etc.? If by 'western civilization" you mean the cultural offspring of Greece circa 2500 years, I doubt gay marriage even ranks in the top 10.
The gradual rendering of gender as obsolete in California over more than a century was one of the main arguments in Judge Walker's opinion on Proposition 8. Gay marriage is on the back part of that process.