The natural, rational objection as to the ontology of what marriage is.
Eliminate the religious aspects and how do you really define what marriage is or should be? The Bible defines marriage as the lifelong union of one man or one woman. Society redefined that as the union of one man and one woman until they decide to end it. Modern society has tried to define it yet again as a union of two adults. Remove religion as the bedrock and how can you say that only the first is a legitimate marriage and the other two are shams?