Every single state in the Union is filled with strange laws, some frighteningly intrusive. To those that simply say "no big deal, these laws are rarely enforced" consider the court case Samples v. Moore from Georgia, 1987.
Ms. Samples and Mr. Moore have been cohabitating for 14 years. When they decided to start living together, they made a straight-forward agreement: they would use her money to provide living expenses and he would save his money for their mutual early retirement. However, he moved out and is now living with another woman. Ms. Samples believes he was playing her the whole time, living off her and saving his money for himself. Fraud is at play.
In order to seal the deal, Samples agreed to start having sex with him. In fact, she indicated that this was a tipping point for him to go along with the agreement. But as it turns out, premarital sex in Georgia is (was?) illegal and "a contract to do an illegal or immoral thing is void." The judge threw out the case.
Even if no police officer enforces it, a law on the books can still ruin your life.