Heterosexual marriage isn't fundamental to our very existence and survival. Making babies is essential, which some heterosexual people do, and raising babies into adults is essential, which some gay people and some straight people do. Marriage isn't necessary for any of it.
Treating people equally isn't fundamental to our existence either, but that doesn't mean we shouldn't do it.
Heterosexual marriages are the best way we currently have of creating babies and raising them well. Once we get a better way I'll buy your argument.
Anyways, that wasn't my point. rms said that law supports gay marriage. But it only support interracial marriage because interracial marriage is also heterosexual. So rms misses the point of the law.
Treating people equally isn't fundamental to our existence either, but that doesn't mean we shouldn't do it.