Jurisprudence professor Hadley Arkes writes in his Meaning of Marriage essay about the train of scary possibilities opened by the logic of Goodridge v. Department of Public Health, the Massachusetts Supreme Judicial Court case that mandated legal gay marriage. The court said marriage law could not be based on procreation, or even on sexual relations. The emotional attachment of same-sex couples was enough of a foundation for marriage under the law.
Arkes points out that, since neither the possibility of children nor the practice of any sort of sex is included in the definition of marriage that the court has now imposed on Massachusetts, any pair or group claiming an emotional attachment could apply for a marriage license there. If they were denied, they could sue, citing the court's own decision to eliminate nearly all grounds of objection. Polygamy and the various grades of adult incest would all have a case that would be hard for that same court to deny.
Beyond that scary possibility, Arkes points out that all the laws and decisions that prohibit discrimination on the basis of sexual orientation, including Goodridge, open a new world of "orientations" to legal protection. The growing consensus that pedophilia is a deep-seated and almost untreatable condition creates the grounds for a lawyer for, say, the North American Man-Boy Love Association (yes, there really is such a thing, and no, they aren't kidding) to argue that their sexual orientation should be protected, too.
The slope is slippery, and the Pandora's box that we are carrying down it is already ajar.