Circumcision does not prevent HIV transmission in any form of sex. Only abstinence and monogamy prevent HIV. The evidence for alleged health benefits of circumcision is pseudoscientific crap, just like that for anthropogenic global warming.
Well, there have been studies conducted that could certainly persuade one to believe that circumcision can prevent HIV transmission in hetereosexuals and not so in homosexuals. E.g.,
http://www.scientificamerican.com/article.cfm?id=circumcision-and-aids
True or not, if homosexuals perceive it as true, then that would make sense as to why Gay Francisco would make this such a priority; though I tend to agree with the poster who said that homos just like the way uncut dicks look.