Unlike Canada, the U.S. does not have a clause in its constitution specifically guaranteeing equality rights; but courts have held that laws which discriminate on the basis of gender or race should be subject to "strict scrutiny"; that usually results in discriminatory laws being struck down.
The case decided today concerned a tax assessment levied when a New York lesbian died, leaving her estate to her spouse. The two had married in Canada and lived in New York where same sex marriage is permitted.
The court held that the widow cannot be forced to pay the federal estate taxes.
This battle parallels Canadian claims for "spousal benefits" which were fought and won for about a decade before same sex marriage was achieved here. Obama has refused to defend the "Defense of Marriage Act" - the homophobic federal legislation denying recognition to same sex marriage. So the widow's opponents were funded by House Republicans in court.