San Francisco men of all sexual persuasions have sharply reduced their number of sexual partners, most likely because of fears created by the AIDS epidemic, according to University of California researchers. UC scientists from Berkeley surveyed 1,034 single men aged 25-54 in San Francisco and asked them to compare the number of sexual partners they had had in the first six months of 1984 with the same period in 1986. Warren Winkelstein Jr., one of the researchers, said the number of homosexual and bisexual men reporting 10 or more male sex partners was down 60% for the periods involved. Similarly, heterosexual men reported an average of 1.8 sexual partners during the first half of 1986, compared with 2.8 during the same period in 1984. Of the bisexual men questioned, only a third reported having female sexual partners. That finding, along with the decline in partners among heterosexual men, should ease concerns somewhat about AIDS transmission from high-risk groups to heterosexual men and women, the researchers reported in the Journal of the American Medical Assn.