The eldest son of Ronald Reagan comes out swinging against gay marriage in a column today using the same slippery slope argument that was also used in the 1960s against interracial marriage.

‘There is also a very slippery slope leading to other alternative relationships and the unconstitutionality of any law based on morality. Think about polygamy, bestiality, and perhaps even murder,’ writes Michael Reagan, the adopted son of the late US president and his first wife, actress Jane Wyman.

He adds: ‘It’s ultimately about changing the culture of the entire country. It inevitably will lead to teaching our public school kids that gay marriage is a perfectly fine alternative and no different than traditional marriage.’

Reagan, 68, is a former conservative radio host whose most recent show was canceled in 2010.

In the column, he also wonders where the moral outrage is as marriage equality gains wider support among the public, the US Congress, and even among some prominent Republicans.

Reagan wonders: ‘Where in the heck are the churches on the issue of legalizing gay marriage? Where are the Protestants, Jews and Catholics? Have they lost their tongues? Their hearts and wills? Their institutional you-know-whats? Where’s the moral outrage? Why aren’t thousands of our pastors, priests and rabbis shouting from their pulpits? Why aren’t they leading their congregations through the streets in mass protest?’