[T]he Second Circuit became the first federal appeals court to hold that laws that classify individuals based on sexual orientation should receive a more exacting level of judicial review, known as “heightened scrutiny.” The First Circuit Court of Appeals in Boston, which ruled DOMA unconstitutional in the Gill case last May, did not apply heightened scrutiny.
[T]oday's opinion is important because the theory underlying the court's holding goes much further than the First Circuit did. The Supreme Court has developed a three-pronged approach to applying the equal protection clause of the Fourteenth Amendment. Under this framework, racial classifications have been held to require "strict scrutiny," under which the classification is constitutional only if necessary to achieve a compelling state objective. Most classifications require merely "rational basis" scrutiny—that is, the state only has to show that a classification bears any plausible rational relationship to a legitimate state interest. In practice, laws rarely survive strict scrutiny and rarely fail to survive rational basis analysis. Gender classifications are subjected to an intermediate level of heightened scrutiny that requires more justification than rational basis but less than strict scrutiny.
As of now, sexual orientation is, at least nominally, a "rational basis" category, and the First Circuit opinion found that DOMA was unconstitutional under that standard.