Every argument supporting gay marriage—‘Love is love,’ ‘we deserve equal protection under the law,’ and ‘we’re not harming anybody’—also supports group marriage. The Massachusetts town of Somerville has become the first in the nation to legalize polyamorous relationships. It’s evidence of the slippery slope social conservatives warned would follow legalizing gay marriage.Polygamy was the obvious evolution of redefining marriage. After all, every argument supporting gay marriage—“Love is love,” “we deserve equal protection under the law,” and “we’re not harming anybody”—also supports group marriage.Somerville’s legal recognition of polyamory came about on June 25 while the city council was changing its domestic...