The Supreme Court ruled that the Legislature was constitutionally allowed to pass the Defense of Marriage Act in 1998. The Legislature felt, at that time, there was a need to protect procreation and encourage families headed by biological parents. This is the argument that was upheld by the court. The decision was NOT a blanket ruling on the constitutionality of gay marriage. In fact, the court ruling specifically stated the Legislature, or the people through the initiative process, could extend marriage rights to gays.
The court stated that they were not going to establish the law on gay marriage. That is a matter for the Legislature to decide. Currently, the Legislature defines marriage as between one man and one woman. The court ruling does not prohibit the Legislature from redefining marriage in the future, if they so chose, to include same-sex couples.