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I experienced the longest attempt to explain how gay marriage somehow harmed those against it about a dozen years ago, when my team and I were getting ready to go live on a project we'd been death-marching for almost fifteen months.
The CIO at this company was a flaming RW ass, with very involved but ultimately contradictory or non-sensical opinions about everything. No boundaries with this guy, so he went off on a week long tangent, throughout our sixteen-hour work days.
The gist was that gay marriage somehow eroded the value of the marriage contract for which the state was acting as third party insuror. Marriage was by definition an arrangement that was exclusive, required investment by its principals, provided a stream of returns, and that by reducing its exclusivity and diluting the pool of those returns provided by or guaranteed by the state (not all of which were material), the value of the overall contract was diminished. Though only a part of the package, this dilution also undermined the social value placed on the institution, deepening the harm. Also, extending (social) benefits produced what economists would call a collective action problem, a tragedy of the commons.
We had multiple go-rounds on why the alleged value outside material benefits was scarce enough for this to apply, what the mysterious essence of this value was. After multiple challenges to other means of 'dilution' by the actions of married heterosexual couples, domestic abuse, divorce, green card scams, polygamy (it came up), etc, what we ended up with was that the value of the contract came from its exclusive nature (he ducked the social reproduction-value question, IIRC), which had to be preserved for there to remain any value in it. Begging the question, in other words, in ten days or less.