It also motivates much of their vehemence in a way that would be hard to explain otherwise. What is a "man" and what is a "woman"? There are, of course, the legal, property, and financial benefits of being married. If marriage only exists for the purpose of having childrenthen how can infertile couples be allowed to marry? Marriage as a Cultural Symbol. Yet even if we accept that it is true, so what?
The same is said even about domestic partnership laws which would give same-sex partners a few of the same basic rights as married couples.
For them, marriage is conceived of almost exclusively in religious terms. Marriage is Sacred and a Sacrament. There is no evidence offered to back this up — not even false evidence, as with the prior argument.
Why would they want to bother?