The burden of establishing that homosexual unions are similarly vulnerable and in need of recognition, as well as being necessary and beneficial to the common good, as heterosexual marriage, is necessarily on those who wish to overturn these warranted claims.
I do not believe it will be possible to establish, based on the evidence detailed above, that such claims are in fact warranted. If the state, nonetheless, adopts such proposals in order to further the political or social agendas of those who cannot establish such warrant, the state would be acting illegitimately, and in opposition to reason. A re-definition of marriage to include same-sex marriage is beyond the competence of the state, because marriage both precedes the state and is a necessary condition for the continuation of the state (because future generations arise from and are formed in marriage).
If the government says that an apple is now the same as an orange, and the law requires everyone to call apples "oranges," the state would have the power to punish anyone who calls an apple an "apple" instead of an "orange," but it would be a totalitarian abuse of raw power and would not change the biological reality of the nature of the fruit in question.