Standing athwart history yelling "Stop!" somewhat louder and more hypocritically than before: President Bush on Wednesday will back a constitutional amendment banning gay marriage in an attempt to halt same-sex unions like the thousands that have been allowed this month in San Francisco. "He has always strongly believed that marriage is a sacred institution between a man and a woman," White House press secretary Scott McClellan said. He said the president wants to end "growing confusion" that has arisen from court decisions in Massachusetts, and San Francisco's permitting more than 3,000 same sex unions. "The president believes it is important to have clarity," McClellan said. He said Bush believes that legislation for such an amendment, submitted by Rep. Marilyn Musgrave, R-Colo., "meets his principles" in protecting the "sanctity of marriage" between men and women.