I believe that our right to privacy justifies a law that prohibits employers from discriminating or firing employees for most off-work behavior and expressions of opinion. Since the US Supreme Court has a mixed record on whether we have a right to privacy, this may take a constitutional amendment. Without such protection and with the trend of corporate consolidation, our constitutional rights could become moot if all or most employers decided to restrict employee's off work behavior.
I just checked California law and its illegal to discriminate against an employee on the basis of martial status. That puts the school in the very difficult position of trying to argue that firing someone for having pre-marital isn't based on marital status.