We conducted a cross-sectional survey of MSM using a time-location-sampling design in San Francisco during 2007-2008. The investigation focused on the selection of sexual partners, partner preferences, perceptions of HIV risk, and social mixing with respect to race/ethnicity. The sample of 1,142 MSM was 56% White, 22% Latino, 14% Asian, and 9% Black and reported on 3,532 sexual partnerships. Black MSM had a significant, three-fold higher level of same race sexual partnering than would be expected by chance alone (i.e., in the absence of selective forces with respect to race among partners). Black MSM were reported as the least preferred as sexual partners, believed at higher risk for HIV, counted less often among friends, were considered hardest to meet, and perceived as less welcome at the common venues that cater to gay men in San Francisco by other MSM. Our findings support the hypothesis that the sexual networks of Black MSM, constrained by the preferences and attitudes of non-Blacks and the social environment, are pushed to be more highly interconnected than other groups with the potential consequence of more rapid spread of HIV and a higher sustained prevalence of infection. The racial disparity in HIV observed for more than a decade will not disappear until the challenges posed by a legacy of racism towards Blacks in the US are addressed.