Once the marriage monopoly is broken up so that different-sex couples have a choice, the competition is likely to last. Where they are available to all couples, experimental new alternatives to marriage have been resilient to change, even after marriage equality is achieved. This has proven true from Belgium and the Netherlands, to Washington, D.C., Hawaii and Illinois, which all recognize gender-neutral marriage alternatives as well as marriage. Apparently, political majorities are hesitant to surrender their existing choices.
In the end, all couples could be left to pick-and-choose among a menu of civil unions, domestic partnerships and reciprocal beneficiary relationships, as well as marriage for both same and different-sex couples. Whether this is a good thing or a bad thing, it certainly cannot appeal to conservatives if they care more about preserving traditional marriage than about showing disapproval of LGBT families. By refusing marriage to same-sex couples, social conservatives are undermining traditional marriage more than lesbian brides and gay grooms ever could.