San Francisco hotel owner and real estate developer Donald E. Werby, 63, at first pleaded not guilty but then withdrew the plea at his arraignment on 21 counts of having unlawful sex with minors and paying for it with money and drugs. Werby, owner of the elegant Four Seasons Clift Hotel, entered the plea in Superior Court but withdrew it on the advice of his lawyer, James Collins, who told the judge that he wanted time to study the transcript of Werby's indictment. Werby, married with three children, came under investigation last year when Roger Boas, former San Francisco city chief administrative officer, was indicted for soliciting sex from teen-age prostitutes. Boas pleaded guilty, was fined $100,000 and sentenced to trash pick up duty. Werby is free on $100,000 bail.