As tptacek says, many states do ban discrimination on the basis of marital status, and they have a good reason to - historically most businesses refused to employ married women at all on the rather sexist basis that they should be at home raising kids.
In my previous job I asked my dev lead from Mumbai to help me with interviewing candidates to join us on a project here in California. On a conference call with a female interviewee, he asked if she was married. He hadn't asked the previous two candidates (males) that we'd interviewed.
After the call I told him that was illegal here and he can't ask that question in an interview. He was shocked. He said Indian women are expected to devote time to their families once they are married. Single women have much more time to devote to the project and would be much better for our team.
I've known that women have been discriminated in this way for a long time, but I had never encountered it personally until then.
