The first is that it's factually debatable. The applicability of studies of African populations to the US is dubious. The studies were not double-blind. Even experts that endorse the argument admit it does not prevent HIV infection, it just makes it slightly less likely, but as it is being sold as HIV prevention it is just as likely to cause increases because those who have had one think it means they can take more risk.
The second problem short-circuits the first.
The second problem with the circumcision prevents HIV argument is that it can be quickly shown to be irrelevant to the core moral issue. The reason circumcision is held to help prevent HIV is that the foreskin contains a large number of Langerhans cells (http://en.wikipedia.org/wiki/Langerhans_cell) and these cells are thought to be more vulnerable to infection. The fact those defending circumcision with HIV reduction claims are either (in most cases) ignorant of or (in the case of experts) simply don't mention is that these cells occur in equal concentration in the analogous female tissues of the inner labia. So if you accept the theory that infant male circumcision reduces HIV and is therefore justified, you must also accept that the same kind of HIV risk reduction would be achieved by removing the inner labia of female infants. But no one would agree the latter is justified no matter how good the science was, and anyone performing it to study its effectiveness would be quickly thrown in prison. If you claim HIV risk reduction excuses infant male circumcision but HIV risk reduction does not excuse similar mutilation of infant females, you are being a hypocrite regardless of the validity of the studies that support the the reduction.
The validity of the studies linking HIV risk reduction and circumcision can be argued. The hypocrisy of considering them valid grounds for sexual mutilation of male infants but invalid grounds for sexual mutilation of female infants cannot be. If you want to avoid being a hypocrite, you must condemn infant sexual mutilation, period, regardless of how socially acceptable a given form of it may be. Please do.