It's complicated. Lets just say that it has to do with the full faith and credit clause. A marriage that is valid in any state is valid in all states conferring upon that couple all rights and privileges that would be available to any other married couple.
If the state courts start finding some constitutional right to homosexual marriage or incestuous marriage or polygamy, then in order to prevent some state like Massachusetts from imposing its homosexual marriage or polygamy laws or child marriage laws on other states, a constitutional definition of marriage is needed. Thus if Massachusetts suddenly decides that it wants homosexual or polygamous marriage, then New Hampshire doesn't have to recognize that marriage.
"It's complicated. Lets just say that it has to do with the full faith and credit clause"
The full faith and credit clause is not really complicated and you described it well. But the more fundamental question that needs to be asked here, which never is, is why have we let government define what marriage is in the first place.
One can ascribe all kinds of esoteric and religious meaning to marriage but when you cut through the hype marriage is nothing more than a contract, legally speaking that is. So the fact is that no American has a right to marriage, they must seek permission from the state.
That is the issue that should be of concern to all freedom loving Americans.
Nice effort but Kerberos is eurotrash. No sense pursuing rational discourse with such as he/she/it. Hasn't had a rational thought since some fascist told him Christianity is evil. Time to invoke the "don't waste your breath" rule.