SAN FRANCISCO -- In this live-and-let-live town, where medical marijuana clubs do business next to grocery stores and an annual fair celebrates sadomasochism, prostitutes could soon walk the streets without fear of arrest. San Francisco would become the first major U.S. city to decriminalize prostitution if voters next month approve Proposition K -- a measure that forbids local authorities from investigating, arresting or prosecuting anyone for selling sex. The ballot question technically would not legalize prostitution since state law still prohibits it, but the measure would eliminate the power of local law enforcement officials to go after prostitutes. Proponents say...