The word “defense” implies an attacker, an infiltrator, an opponent. In this case, the implied attackers are gay people who want to marry each other. So the Defense of Marriage Act defends marriage from people who want to be married. Great work so far, but there’s more:
Since marriage isn’t a person you can torture or a landmark you can destroy but a thing people do, the most obvious way to attack it would be to stop people from doing it. Right?
DOMA doesn’t exactly stop people from getting married, but it does allow states to strip married couples of their rights. Only gay couples, though. Not straight ones.
The Defense of Marriage Act doesn’t defend marriage. It defends bigots. But the Defense of Bigots Act never would have made it to the president’s desk.