Perhaps you have already read about the polyamorous “throuple” of lesbians who have “married” and who are expecting their first child. If you support gay marriage, on what principle could you possibly oppose the “marriage” of three or more people? Robert George’s brief analysis is spot-on:
The story of a female throuple in Massachusetts (with a baby on the way) provides further confirmation, as if any were needed, of the proposition that “ideas have consequences.” Once one has abandoned belief in marriage as a conjugal bond (with its central structuring norm of sexual complementarity) in favor of a concept of “marriage” as a form of sexual-romantic companionship or domestic partnership (“love makes a family”), then what possible principle could be identified for a norm “restricting” marriage to two-person partnerships, as opposed to polyamorous sexual ensembles of three or more persons?… No one has been able to answer the question or meet the challenge.
He’s absolutely right. Read the rest here.