Laws banning cohabitation existed, I would expect, in every state in the union in early America. They existed long before the Nanny State. The Nanny State was brought to us by people like the ACLU, who applauded the court for striking down the cohabitation law.
If you're a conservative, where would you rather live in terms of the current government and political atmosphere: Sweden or South Carolina? HINT: There's not the chance of a snowball in hell of a law against fornication passing in Sweden.
In the USA we've allowed our zealots to operate under the 'victorian compromise'.. -- IE, you can have your blue laws [and even occasionally enforce them - on the lower sorts] as long as you let the rest of us pursue life & liberty.
Banning 'cohabitation' is, in this case, interfering with liberty.
I would rather live in freedom which neither of the examples you gave adequately respect.
Who my neighbor sleeps with is none of my business so long as both are of the age of consent. There is a virtue in minding one's own business.