The Fifth Amendment includes a guarantee that "No person shall... be deprived of life, liberty, or property, without due process of law".
"Due process of law" is the key here; the government cannot simply say "well, we don't like this person, so we're just going to decree arbitrarily that he doesn't get the same rights as other people".
What it actually means will vary depending on what the government is trying to do; in a criminal case, for example, due process includes all the procedures and safeguards to make sure you get a fair trial before you can be thrown in prison.
Of course, DOMA is more or less literally the federal government saying "we don't like these people, so we're just going to decree arbitrarily that they don't get the same rights as other people".
The Fifth Amendment is the relevant one here because the first batch of amendments -- the Bill of Rights -- are traditionally interpreted as only restraining what the federal government can do; they do not, it is generally believed, constrain the states. The Fourteenth Amendment added equal-protection and due-process requirements for the states, so when a state does something that treats people unequally the Fourteenth Amendment is what the resulting lawsuit usually turns on.
"Due process of law" is the key here; the government cannot simply say "well, we don't like this person, so we're just going to decree arbitrarily that he doesn't get the same rights as other people".
What it actually means will vary depending on what the government is trying to do; in a criminal case, for example, due process includes all the procedures and safeguards to make sure you get a fair trial before you can be thrown in prison.
Of course, DOMA is more or less literally the federal government saying "we don't like these people, so we're just going to decree arbitrarily that they don't get the same rights as other people".
The Fifth Amendment is the relevant one here because the first batch of amendments -- the Bill of Rights -- are traditionally interpreted as only restraining what the federal government can do; they do not, it is generally believed, constrain the states. The Fourteenth Amendment added equal-protection and due-process requirements for the states, so when a state does something that treats people unequally the Fourteenth Amendment is what the resulting lawsuit usually turns on.