Marriage in the United States shall consist only of the union of a man and a woman. Neither this Constitution or the constitution of any State, nor state or federal law, shall be construed to require that marital status or the legal incidents thereof be conferred upon unmarried couples or groups.
This is the text of a proposed amendment which began to be seriously considered by some in the US government in 2003 and was referred to by George W. Bush in his 2004 State of the Union address. It is clearly intended to import into the US Constitution a bias in favour of heterosexuality and against homosexuality by restricting the sanctified word "marriage", and the privileged condition of marital status to the publicly recognised display of the former.

As such it is a particularly blatant salvo in the "culture wars" which have set Americans against each other in curious attempts to curtail each other's freedom of speech, freedom of action in cases where no harm is being done to others, equality before the law, etc. etc..

What its promulgators seem to be desirous of preventing is something like the following scenario. A homosexual couple find a church willing to give them a ceremony of marriage, or a city authority willing to issue them a marriage licence (really, I am hopelessly ignorant about how such things are done, but please bear with me). Subsequently the government refuses to recognise the marriage for tax purposes, or in some other context where the legal status of marriage makes a difference. Or the couple in fact fail markedly in their request for the legal form of marriage from the civil authorities. Or such a couple move in from Canada or Vermont where they are recognised as married. (Not sure about Canada, but it might happen.)

Then a lawsuit is brought under the principle of equality before the law, the refusal to recognise or grant the legal benefits of marriage being a material instance of unequal treatment. Then it goes up through the American court system, probably as far as SCOTUS, and the judges decide either that the Constitution allows this sort of discrimination, or that it does not. And who can trust these pesky activist judges? They might just make the wrong decision. The Defense of Marriage Act is a previous attempt to discriminate against gay marriage, which might indeed end up being ruled unconstitutional. So apparently we need this constitutional amendment, which can't suffer the fate of being struck down under the Constitution.

However, if passed, it will have the unfortunate effect of preventing any future marriages (whether heterosexual, homosexual, polygamous, polyandrous or of other varieties) from being legally recognizable at all. Read it carefully. If you are unmarried when the amendment comes into effect, you can of course get married in accordance with state law. But then the amendment says that if you try to uphold your marital status in any court whatsoever, the judge is bound to rule against you, even though you are as heterosexual as the most ardent Puritan might require. Whoever is challenging your marriage in court can point to evidence that you were once unmarried. Then you would want to point to a licence issued correctly under state law, which confers upon you marital status and "the legal incidents thereof". But the judge is not allowed to construe the state law in such a way that it would have conferred marital status on you, seeing as you were unmarried at the time.

So the amendment as written, far from 'defending' the current widely, though not universally accepted definition, would lead to the utter extinction of the legal state of marriage. Besides, in so clearly mandating unequal treatment before the law, it imports a self-contradiction into the Constitution. I think it was Pal Erdös the mathematician who claimed to have found such a contradiction already, while studying the Constitution as a requirement of taking up US citizenship. It would be entertaining, if nothing else, to see the Supreme Court trying to rule on the basis of a self-contradictory document.