I’m not quite sure how a cabbie would determine “marital status” or “family responsibility” or “political affiliation” or “source of income” or other non-visible qualities. I do know that if one asked me about any of these topics he wouldn’t need to discriminate against me — I’d never get into a taxi with a complete stranger who asked me such intrusive personal questions. (It’s nerve-wracking enough to trust that complete stranger to drive you to your destination without incident, without wondering whether the personal inquisitiveness means he is a complete nutcase, if not an axe murderer.)
Although the list of protected characteristics is long, it is not exhaustive. It appears D.C cab drivers could still refuse to transport someone who smells awful, or displays visible signs of complete insanity, or is brandishing a hand grenade.