God defines marriage. It is an eternal institution. Whether or not man accepts it or to what extent people are willing to respect it is an other matter. But when a society rejects God sufficiently, God reject it. In the case of a society that claims to know God, but brings His name into disrespect because of how they trample on His values, God will punish them for their actions.
Perhaps, but religious arguments do not help your cause. The formation of people into exclusive male-female mating pairs is not a consequence of any religious teaching, and arguing from a religious perspective will make it easier for non-religious people to ignore the unassailable secular basis of the institution.