Those who defend the practice of circumcision tend to fall into two camps. The first argue that to forbid it is to violate the religious freedom of the groups (mainly Jews and Muslims) who practice it. This is absurd. No one is saying that people should be forbidden from getting circumcised. What we are saying that circumcision should not be forced on anyone else. If any adult wants to get circumcised, they should be allowed to do so. But to allow parents to mutilate their babies to satisfy their own religious beliefs is simply wrong. You shouldn’t be allowed to establish your bona fides with god by mutilating your children.
The other reason given in favor of circumcision is the supposed health benefit. The one that is most touted is supposedly enhanced defense against AIDS transmission. This was never a convincing argument since babies don’t have sex. Even if it were true, people could choose for themselves to be circumcised later in life when they become sexually active.
But thanks to commenter Jeff Hess, I came across this article (via Andrew Sullivan) that debunks this whole idea of circumcision being an AIDS inhibitor, saying that is based entirely on bad science and that the supposed benefits are not only non-existent but the practice may actually increase the risk of AIDS transmission.