Fox News contributor and conservative columnist Cal Thomas promoted the popular right-wing talking point that marriage equality would create a slippery slope toward polygamy and marriage with children, despite the fact that this myth has been consistently disproven.

In a March 12 column for FoxNews.com on “what we ought to be asking gay marriage advocates,” Thomas posed the question - if same-sex marriage is legalized, what reason can be given for denying polygamous marriages or marriages with underage children?: 

What advocates for same-sex marriage should be asked is whether they consider any other human relationship worthy of similar constitutional protection and based on what standard? The Constitution doesn't guarantee the right to marry. States, not the federal government, issue marriage licenses. 

Current laws restrict “underage” marriage, as well as polygamy. If same-sex marriage is approved, what's to stop polygamists from demanding legal protection and cultural acceptance?Justice Antonin Scalia predicted as much in 2003 in his dissent of the Lawrence v. Texas case, in which the Court struck down the sodomy law in Texas. So I ask, if “fairness” and “equality” are the standard, isn't it also “unfair” to “discriminate” against polygamists who wish to live in “loving” and “committed” relationships?

Since we are rapidly discarding the rules for living and social order set down in a book found in most motel room drawers, what is to replace it? Opinion polls? Clever legal arguments? Fairness? What exactly does “fairness” mean and who decides what's fair? Many things may seem “unfair,” but not all can, or should, be addressed by courts. [emphasis added]

The slippery slope argument has been peddled by conservative and anti-gay activists since the beginning of the debate over same-sex marriage, and it remains just as deeply flawed as it's always been. 

1. Polygamous And Child Marriages Harm Society. The most obvious problem with Thomas' argument is that there are strong and compelling reasons for opposing polygamous and child marriages. Studies have found that polygamous marriages typically cause a host of problems for the men, women, and children involved in them, and young children aren't ready to make informed decisions about lifelong commitments and raising families.

Same-sex relationships, on the other hand, are beneficial to the adults involved in them, as well as the children raised by these couples. There's no compelling reason to deny legal protections to families headed by same-sex couples. 

2. Polygamous Marriage Isn't A Basic Civil Right. When gay people are prohibited from marrying, they are denied equal access to an institution available to heterosexuals simply because of a deeply ingrained, immutable part of their identities. Gay people are barred from benefits, rights, and privileges afforded to heterosexual couples simply because of the way they were born.

This is not the case with polygamous marriages. Though some people may prefer polygamous relationships, this refusing to accommodate this preference is different from denying an entire group of people access to a civil institution based on who they are. As Andrew Sullivan wrote: 

Almost everyone seems to accept, even if they find homosexuality morally troublesome, that it occupies a deeper level of human consciousness than a polygamous impulse... And almost all of us tacitly assume this, even in the very use of the term “homosexuals.” We accept also that multiple partners can be desired by gays and straights alike; that polygamy is an activity, whereas both homosexuality and heterosexuality are states.

3. The Slippery Slope Has Been Disproven Over And Over. Though conservatives have been warning about the slippery slope for years, there's no empirical evidence that legalizing same-sex marriage results in further “redefinition” of marriage. A high court in Canada, for example, recently upheld the country's ban on polygamy, despite the fact that gay marriage has been legal in Canada since 2005. As Lauren Ramseyer at Pam's House Blend recently noted:  

Not a single country that has legalized civil marriage for same-sex couples has gone on to legalize polygamous marriage.  Countries that have legalized civil marriage for same-sex couples are Argentina, Belgium, Canada, Denmark, Iceland, Netherlands, Norway, Portugal, South Africa, Spain and Sweden.

On the contrary, there is a great deal of overlap between countries that criminalize homosexuality or same-sex marriage and countries that have legalized polygamous marriage. [emphasis original]