Though President Obama endorsed gay marriage last week, he maintained that the definition of marriage is an individual state issue. But if several recent court cases are any indication, the decision on gay marriage may ultimately fall to the U.S. Supreme Court.

The first of these cases involves Proposition 8, a 2008 California ballot initiative overturning a California Supreme Court decision that legalized gay marriage. Federal District Judge Vaughn Walker struck down Proposition 8 in August 2010, a decision that the 9th Circuit upheld in a 2-1 vote this February. Supporters of the measure have asked a full 9th Circuit panel to review the case. If the court declines, the case will likely head to the Supreme Court.

The Defense of Marriage Act (DOMA), a 1996 law denying federal recognition to same-sex marriages, has also been the subject of recent legal battles. In Massachusetts, the first state to legalize gay marriage, a federal district court judge ruled in favor of seven couples and three individuals suing for federal marriage benefits. Judge Joseph Tauro said in his ruling that DOMA violates the constitutional right to equal protection under the law. An appeals court is currently deciding whether to uphold that 2010 ruling.

The question of federal benefits could have implications for employers. The Family and Medical Leave Act (FMLA), for instance, does not currently cover employees who take time off to care for their same-sex partners (though it does cover leave to care for the children of same sex-partners). Additionally, same-sex partners of federal employees do not receive the health insurance that would be granted to heterosexual married couples.

Gay marriage supporters say that bans on same-sex unions violate the 14th Amendment’s equal protection and due process clauses. The Supreme Court has adopted this argument in other marriage-related cases, notably ruling in Loving v. Virginia that state laws barring interracial unions were unconstitutional.

Opponents of gay marriage, however, argue that the states have banned other types of marriages—such as bigamy, incest and child marriage—without violating constitutional rights. Judge N. Randy Smith, for instance, dissented from the 9th Circuit’s ruling on Proposition 8, saying that governments have an interest in “a responsible procreation theory, justifying the inducement of marital recognition only for opposite-sex couples.”

Read more about the legal issues surrounding gay marriage in The Wall Street Journal.