Now, in all honesty, there is no rational reason to prevent more than two consenting adults from getting married. The only reason to deny multiple partner marriages is the xenophobic nature of men like O’Reilly. These are men who often push the idea of American Exceptionalism and traditions without actually ever defining either except in concept of White Privilege.
Whether or not there is a “constitutional right” for anyone to get married could be up for debate, but what is not up for debate is that the states have always decided the issue of who gets to marry whom. This is why there is no uniform age of consent in the country, and no uniform ban on consanguinity. In some states, it is illegal for two first cousins to get married, for instance, or for someone who is 50 to marry someone who is 16 without parental consent.
Once same-sex marriage became legal in Massachusetts, the United States government should have recognized those marriages right off the bat, and all the states should have been required to recognize those marriages as well.
Now, there is a precedent that O’Reilly is ignoring, and it comes via Loving v. Virginia. Chief Justice Earl Warren wrote that
The freedom to marry has long been recognized as one of the vital personal rights essential to the orderly pursuit of happiness by free men.
Marriage is one of the “basic civil rights of man,” fundamental to our very existence and survival. Skinner v. Oklahoma, 316 U.S. 535, 541 (1942). See also Maynard v. Hill, 125 U.S. 190 (1888). To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive of the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State’s citizens of liberty without due process of law. The Fourteenth Amendment requires that the freedom of choice to marry not be restricted by invidious racial discriminations. Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State.
Thus, for O’Reilly to argue that anyone does not have a right to get married within certain very narrow parameters is for the man to be incorrect in his assessment of the situation. The thing is, O’Reilly just does not have an adequate argument that falls within the bounds of the Constitution as to why the Federal Government and the Constitution should not apply to LGBT Americans. What he has is a religious one, but he is also smart enough to know that trying to argue from a Catholic perspective is going to get him into deep trouble in the short run.