At one time it was illegal in EVERY state for adult lovers to live together without being married. Today, on the other hand, just 6 states (Mississippi, Virginia, West Virginia, Florida, North Dakota, and Michigan) still criminalize cohabitation by opposite-sex couples, although they are generally not enforced.
I know that the ACLU had a lawsuit pending before the Michigan Supreme Court back in December '05 on behalf of a father who was being denied Parenting Time because he was "cohabitating". If girlfriend was forced to sleep elsewhere when the had the kids.
If you air your dirty linen in public, expect people to comment on the skid marks!