Perhaps not today (I'm unfamiliar with current federal laws on this), but for a considerable portion of US history the US federal government took your property and threw you in jail.

http://en.wikipedia.org/wiki/Morrill_Anti-Bigamy_Act http://en.wikipedia.org/wiki/Edmunds_Act http://en.wikipedia.org/wiki/Edmunds%E2%80%93Tucker_Act

Not true. Merely living together with your partner, if not legally married, can be a crime. E.g., punishable by 60 days in jail in Florida.

http://articles.cnn.com/2011-09-01/us/florida.cohabitation.l...

That law is invalid, so not a big deal.