What is really at issue here--IMO, at least--is adoption. If we allow homosexuals to call their "thing" marriage, we might assist in opening the door to adoption of children by homosexuals.
You might think of "marriage," as involved here, as a trademark. The state has decided to make the use of this "trademark" a requirement for adoptions that are organized by the state. Therefore, the state is a party to a marriage contract in a way.
I don't object to homosexuals doing whatever they please with each other, as long as they do it on their own property and no one can see or hear them. But they shouldn't call it a marriage, because that's not what it is.
That also goes a long way into explaining why homosexuals want their "lifestyle" to be taught in public schools.