I don't understand why it isn't. My Bible says God said to go forth and multiply, and I'm sure he meant under the covenant of marriage, therefor, it is God-given and inalienable.
Restrictions can, and have been used (must be old enough, can't marry your sibling, etc.), and for good (obvious) reasons. But the Constitution doesn't give us ANY rights. That's not its design, although the liberals have obviously taken that approach.
We heteros need a better argument to prevent homos from re-defining the word marriage.