Four years ago, the two us of joined forces and launched a federal constitutional challenge to Proposition 8, a ballot initiative which eliminated the right of same-sex couples in California to marry. Tuesday, the Supreme Court will hear argument in that case. The central question is whether a state may exclude gay and lesbian Americans from what the Supreme Court has called "the most important relation in life"—the institution of marriage. The answer is no.

We represent two loving and committed couples. In many ways,...