Congress is the space for majorities to struggle for the distribution of social gains, not the place for majorities to decide on minorities' rights. Majorities have too much of an incentive to exclude minorities and violate fundamental rights, and one should always be suspicious of the will of the many. In a liberal democracy, Courts are there to protect those rights, especially to stop majorities in Congress when they trespass to the area of individual and minority rights.
If majorities are asked about gay marriage, for example, in almost every country of the Americas they would condemn it. See this study. The will of the people would be against the rights of minorities, as it often is, and gay couples will not be full citizens. This question is not about the distribution of social gains; it is about the fabric of society, its fundamental right of equality.
