SAN FRANCISCO, Feb. 24 — As President Bush declared his support for a constitutional amendment banning gay marriage, the political and legal landscape surrounding the marriage licenses issued here to same-sex couples appeared rockier than ever on Tuesday. Mayor Gavin Newsom, whose city was singled out by the president, held a news conference at which he called Mr. Bush's support for an amendment "shameful" and "cowardly" and accused the president of a "lack of truthfulness." Mayor Newsom, whose directive to the county clerk on Feb. 12 has resulted in more than 3,200 same-sex marriages in San Francisco, said Mr. Bush's...