Cohabitation Law?

Hello, is it against the law for my ex’s girlfriend to be living in the house? My kids are there only 50% of the time (every other week) & they are planning to get married but I thought it is not allowed by law for two people to live together w/o being married. I know this sounds old fashioned but I am trying to raise my 16 year old daughter not to be the next hoochie generation and the girl is sleeping in his room while they are there. Thank you in advance for the information.

No, it is not illegal for them to be living together.
Adultery (actually having sex with someone other than your spouse) is illegal, however it is up to the prosecutor to decide if they wish to prosecute the case.