Only four states have legalized same-sex marriage, and it's pending in just a handful of others. Forty states limit marriage to a man and a woman. But public opinion supports civil unions for gays and lesbians, and voters aged 18 to 45 are favorably inclined toward homosexual rights. Does the Constitutional guarantee of equal treatment include same-sex marriage? Are religions free to discriminate as they choose? How does the issue work for — and against — Democrats and Republicans?