system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:  What's DOMA and why is it considered unconstitutional?
gpt
There are a couple of arguments, mostly to do with ennumerated powers, equal protection and full faith and credit.

The 10th Amendment says in part that any power not specifically given ('ennumerated') to the federal government by the Constitution belongs to the individual states or to the people. As the regulation of marriage isn't at all related to any of those powers, the argument is that DOMA is unconstitutional by that reason alone; in other words, Congress never had the authority to make any law about marriage, that being the job of each state to decide for itself.

Article 4, Section 1 of the Constitution says essentially that each state must recognize as valid the laws of all the other states; in this case, essentially meaning that if you get married in Massachusetts, Kentucky isn't allowed to act like you're not married when you visit Louisville. This is why opponents of gay marriage care what other states do, since they're legally prohibited from just ignoring it.

The 14th Amendment says in part that the states are required to apply all laws equally to all people. The way it's written, this doesn't apply to the federal government, so DOMA itself has been hard to challenge based on equal protection so far, but it has been an obstacle to state-level laws banning gay marriage.