For a cohabitation agreement to be valid, it must be: 

1. In writing; 
2. Signed by both parties; 
3. Properly notarized; and,
4. Agreed to in good faith.

Furthermore, since prostitution is illegal, any cohabitation agreement made in consideration of sexual services is likely invalid.

New York courts have sometimes upheld oral and “implied” contracts between cohabitating partners. However, these agreements are far less secure, and would require a court action to enforce. The financial and emotional cost of that court action may likely make it not worthwhile; especially given the limited prospects of success. Therefore, it’s always better to outline important issues in a written cohabitation agreement.