An HIV-positive man who admitted having sex with his 8-year-old stepdaughter over a two-month period was sentenced in a San Mateo County courtroom today to 16 years in prison. Frederick Torralva, 53, pleaded no contest today to one count of engaging in three or more acts of substantial sexual conduct with a child under the age of 14, the San Mateo County district attorney's office reported. He also pleaded no contest to the special allegation that he had knowledge that he was HIV-positive at the time of the crimes against his wife's child,