“The right to marry whoever one wishes is an elementary human right compared to which ‘the right to attend an integrated school, the right to sit where one pleases on a bus, the right to go into any hotel or recreation area or place of amusement, regardless of one’s skin or color or race’ are minor indeed. Even political rights, like the right to vote, and nearly all other rights enumerated in the Constitution, are secondary to the inalienable human rights to ‘life, liberty and the pursuit of happiness’ proclaimed in the Declaration of Independence; and to this category the right to home and marriage unquestionably belongs.”