Prager's prediction dovetails with those of other marriage equality opponents who similarly suggest that necrophilia and bestiality might become commonly accepted practices if gay couples are allowed to marry. But in the 10 years since Massachusetts became the first state to legalize marriage equality, there hasn't been a rush to legalize polygamous unions. Meanwhile, most states that allow incestuous marriages are right-leaning states where same-sex marriage currently isn't allowed.
As Slate's Dahlia Lithwick has observed, the problem with "slippery slope" arguments of the kind advanced by Prager is that they ignore the deep differences between allowing a committed, loving same-sex couple to get married and permitting, say, a brother and sister to get married. Incestuous relationships, Lithwick notes, are often exploitative and psychologically destructive, with severe consequences for children's health.
The response to bigots when they bring up polygamy and consanguinamory is "What's wrong with letting consenting adults marry?" Please note that under our broad legal systems, corpses and other species (necrophilia and bestiality) are not considered consenting adults. However, a consenting adult might want to marry more than one person, or marry a close relative. When (white) women got the right to vote, there wasn't a rush for voting rights for people of color, and when Loving v. Virginia knocked down bans on monogamous interracial marriages, there wasn't a rush to grant to same-gender freedom to marry, but there should have been.
It is unfair to say that incestuous relationships are often exploitative and psychology destructive. That is ABUSIVE relationships in general, which can include complete strangers and interracial couples same-gender couples. Also, it is the abusive relationships that tend to come to the attention of law enforcement and counselors. Nobody in a good relationship is calling up a shrink or law enforcement and saying, "Hey, I just want to tell you I'm in an incestuous relationship and it is great!"
The "mutant baby" argument is a smokescreen. First of all, some consanguinamorous relationships involve only people of the same gender. Yes, they are gay marriages, so to speak. I have interviewed people in these relationships myself. Secondly, marriage shouldn't be equated with baby-making. Not all mixed-gender relationships birth biological children. Thirdly, contrary to myth, most births to consanguineous parents do not produce children with significant birth defects or other genetic problems (I know some of these children, and so do you, whether you know it or not); while births to
other parents do sometimes have birth defects. Heterosexual couples with obvious, series genetic diseases are not prevented from dating, having sex, having children, or marrying, so the "mutant baby" argument is not a justification for stopping genetic half-sisters who didn't even grow up with each other from marrying.
I expect more from Media Matters than to throw some consenting adults under the bus to assuage bigots. There is no good reason to deny that we must keep evolving until an adult, regardless of gender, sexual orientation, monogamy or polyamory, race, or religion is free to marry any and all consenting adults. The limited same-gender freedom to marry is a great and historic step, but is NOT full marriage equality, because equality "just for some" is not equality. Let's stand up for EVERY ADULT'S right to marry the person(s) they love. Get on the right side of history!