New Jersey and New Hampshire also recognize civil unions.
Last week, the Iowa Supreme Court ruled that not permitting gay marriage in the state was unconstitutional.
Last fall, California voters reversed a court decision that legalized gay marriage when they approved a referendum known as Proposition 8.
The District passed a law in 1992 that allowed same-sex couples to register as domestic partners. But Congress prohibited the city from spending any of its own funds to implement the law, until the ban was finally lifted in fiscal 2002.
The recognition entitles domestic partners to benefits that can include health insurance coverage and mutual visitation rights in hospitals and nursing homes, and generally equates a domestic partner with a spouse in sections of city law.
New York, New Jersey, Illinois, and Washington are among at least six states that have same-sex marriage measures pending in their General Assemblies. However, none is expected to pass.