State law, typically, but I think there is some federal law covering it, too.
I had to go look up the federal laws to see if married/not married is a protected status. Apparently not, if you apply the same measure to both men and women.
TITLE VII - CIVIL RIGHTS ACT OF 1964 (42 USC 2000e AND 12102 ET. SEQ.)
Title VII is the primary federal employment antidiscrimination legislation. It applies only to employment except concerning individuals with disabilities.
PROTECTED GROUPS:
•Race
•Color
•Religion
•Gender (including pregnancy and marital status if applied only to one sex)
•National origin or ancestry
•Disability
Age Discrimination
AIDS/Disease
Civil Rights
Disabilities (ADA)
National Origin Discrimination
Racial Discrimination
Religious Discrimination
Sexual Orientation Discrimination
Vietnam Vets/Veterans