HOUSTON — A federal judge in Texas struck down the state’s ban on same-sex marriage on Wednesday, ruling that the laws restricting marriage to a man and a woman violated the United States Constitution, handing gay-rights advocates a major legal victory in one of the nation’s biggest and most conservative states. The judge wrote that the amendment to the state Constitution that Texas voters approved in 2005 defining marriage as between a man and a woman — and two similar laws passed in 1997 and 2003 — denied gay couples the right to marry and demeaned their dignity “for no...