Fox News contributor Erick Erickson hyped the claim that legalizing same-sex marriage would pave the way for same-sex incestuous marriages, claiming that “many, many, many” marriage equality supporters will support incest and polygamy once “they can shift public opinion further.”

In an April 5 blog post on RedState, Erickson echoed controversial comments made by actor Jeremy Irons, who criticized marriage equality by suggesting fathers would marry their sons in order to avoid paying estate taxes. Erickson agreed, arguing that “there is little moral difference” between loving, committed gay couples and incestuous relationships:

Seriously. Why not incest. [...] If love and commitment are the justification for marriage, why exempt this? [...] So why not fathers marrying sons and moms marrying daughters? Is it because of the “ick” factor? Why should that preclude it? If life comes down to who you love and who loves you back, if a father and son love each other so much they want to get married, there is little moral difference between two people of the same sex getting married who are not related and want to be and two people of the same sex who already are related becoming closer. [...] The truth is, many, many, many of the same people who are now in support of gay marriage, but would oppose this or polygamy will, once the next step is advanced, support these things too. They just have to lie about it for now until they can shift public opinion further.

Erickson's argument is riddled with the same flaws that have always plagued the conservative slippery slope argument against marriage equality.

1. It's Empirically False - In the states and countries that have legalized same-sex marriage, there's been no evidence of a rush to legalize or destigmatize incest. In fact, most of the states that allow for marriages between first cousins are conservative-leaning states with explicit bans on same-sex marriage.

2. Incest Causes Real Harm To Children - Unlike in the case of same-sex marriage, there are persuasive reasons for banning incestuous marriages. Romantic relationships between parents and their children are typically exploitative and psychologically damaging. As Slate's Dahlia Lithwick wrote:

The problem with the slippery slope argument is that it depends on inexact, and sometimes hysterical, comparisons. Most of us can agree, for instance, that all the shriekings about gay marriage opening the door to incest with children and pedophilia are inapposite. These things are illegal because they cause irreversible harms.

There are plenty of compelling arguments for opposing marriages between parents and their children. “Gay people shouldn't be allowed to marry, either!” isn't one of them.