How appropriate. Gavin Newsom, San Francisco's mayor, this week lost a key aide after having an affair with the man's wife. Man-about-town Newsom's new girlfriend is Jennifer Siebel, an extremely attractive blonde actress. Siebel's next movie? The appropriately titled 'Trouble with Romance', which premieres at the Cinequest festival in a month. The photo opp Newsom's remaining staffers will want to avoid: a picture of San Francisco's accident-prone mayor in front of the movie's poster.