Here's how "it would work".
The foundation of a civilization is the nuclear family. Redefining "family" until it is unrecognizable undermines this foundation, exacerbating certain destructive trends. "Family" has always been held as different, both "above" and more fundemental, than civil law, and thus has been the only stable platform on which to build a civil society. Government recognition of homosexual trysts as "marriage" further destabilizes and delegitimizes this unique role of the family in a society. Will "same sex marriage" singlehandedly bring down Western Civilization? No. Is it another nail in the coffin? Most certainly.