If “marriage” isn't in the Constitution, YOU DO NOT HAVE A RIGHT TO IT!
I have to disagree with you here SFD. The Constitution is primarily a limit on Government.
The tenth Ammendment states "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people"
We have 'rights' to everything that is not mentioned in the Constitution. However, that doesn't mean that we have the right to recognition of some of those things by government (or anyone elsefor that matter)
So we have the right to form unions between two people. We do not have the right to have those unions recognised (as marriages or any other way).
Recognition of marriage by the government is tied to lots of programs and financial incentives for the purpose to "secure the blessings of liberty to ourselves and our Posterity"
If there's no chance of posterity (children) then why recognize the marriage at all. While a union of any two mentally healthy people (of opposite sexes) have at least the chance of producing children, no 'homosexual' union can do so.