It's more than even that. The concept of "marrying" two people, things, whatever (it's a term used in a variety of contexts, including cooking, i.e., to "marry" the flavors of various ingredients) means to mix and combine two different things.
You cannot "combine" two same things.
Now, consonant with the concept of "marriage" among human beings is the concept of "heterosexual". The "hete" root is the key. Heterosexual means something. What it means, is that a heterosexual is attracted to and mates with someone of a different gender.
The two concepts -- heterosexuality and marriage -- are part of the same story. Two views of the same picture. You cannot take one away and maintain the other. (Even when using terms like "common law" marriage.)
To -- by fiat -- declare that two same-sex individuals can "marry" is tantamount to declaring that the law of gravity is null and void within the boundaries of San Fransisco. Or that people shall be allowed to watch television on their Etch-a-Sketch toys.
The law can demand it. People can even go through the motions of pretending to "enjoy their newfound rights. But nothing will be changed, other than to institutionalize Orwell's doublespeak.
And you can't do that without creating a type of mass insanity.