The following is Section 1 of the 14th Amendment to the US Constitution.
Section 1. All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
Now, I can understand why some people, especially religious groups are opposed to the idea of gay marriage. I think they're wrong, but that's not the point of this. Here in the rather questionable state of Missouri, they just passed a marriage definition into the state constitution and in doing so, have created a consequece I don't think they ever considered.
Under the US Constitution, no one can make a law (in this case amandment to the state constitution) that abridges the privileges of any citizen. Because the government recognizes marriage, the recent action means gay people no longer are eligible for it. Well they are, but we'll deal with part 2 of this mess in a minute. Part 1 is, they can't marry the people they wish to marry. Under that standard, the government now has two options. They can either strike down the state amendment, restoring the primacy of federal rule, or, they have a second option, and this gets even more convoluted, they can strike marriage as something no longer recognized by the government.
There's also a third option which is part 2 of this and should be much much more entertaining, which is of course the sham marriage, but that's not really an option.
So why is one of the first two ideas what will have to happen? Well, it says right there in the "bible of government" that everybody is equal under the law, as eligible for any any right or privilege they give as anyone else. Now they can place some sorts of restrictions on these, and have. You have to be a certain age to get a driver's license and if you do certain things, they can take it away, but everyone is eligible until they do something to make themself ineligible. If you follow the rules, everyone can do everything under the law.
So, what happens now is anyone's guess. The only two options are strike the rule, or abolish government recognized marriage. Marriage is a mostly religious institution anyway, so maybe what the step is, government can only recognize civil unions, and leave marriage to the church. (The IRS would go nuts trying to fit that on a 1040, but no one likes the IRS anyway) I don't think that solution makes anyone very happy because it means changes for all kinds of people. Insurance companies would be forced to retool all their rules because just like the government, companies can't discriminate on religious grounds (which marriage would now be) and states would not be able to do both marriages and civil unions because that falls into the category of "separate but equal" which was struck down under Brown v Board of Education.
I would deeply like to thank the current administration for this pile of crap. They've chosen to make screwing over hate and discrimination a centerpiece of American daily life and are doing their best to make it the law.