WHEN THE Massachusetts Legislature meets in joint session as a constitutional convention this week, the most notable item on its agenda will be a proposed amendment to ban same-sex marriage. A record-breaking 170,000 registered voters have signed petitions to put such an amendment on the state ballot. But the Massachusetts Constitution mandates a detour: The measure must first win the support of at least 50 lawmakers in two consecutive legislative terms. Only then can it be submitted to the people. If the amendment gets past every hurdle, it will reach the ballot in November 2008. It is a deliberately long...