Circumcision is a specialized surgery wherein a portion of the genitalia is amputated or mutilated; in boys, it's usually a significant portion of the foreskin, and in girls, it's usually a significant portion of the vulva.
Yes, unless sufficient anaesthetic is administered to the patient beforehand. Unfortunately, this is not an option for infants, who have to be physically restrained so they do not injure themselves when reacting to the pain.
While anaesthetic is a comparatively new invention, the practice of not administering anaesthetic in the ritualistic circumcision of infants and young children unfortunately persists even today.
No, although many medical professionals are opposed to non-therapeutic circumcision being performed on children for safety and ethical reasons.
When circumcision is imposed on a person without their consent (infants and young children cannot be adequately informed to give consent), that's normally a violation of their human rights.
See also: Article 3 of the Universal Declaration of Human Rights (UDHR): "Everyone has the right to life, liberty, and security of [the] person."
Circumcision, like any surgery, is not risk-free, and in many cases has been fatal. Non-therapeutic circumcision should not be performed on children of any age.
Acts of sexual intimacy is less satisfying because the amputation of tens of thousands of nerves results in severely reduced sensitivity.
No, circumcision does not prevent or cure diseases. Over the past century many fraudlent claims have been presented by dishonest circumcision practitioners and proponents to trick people into circumcising their newborn babies, but the reality is that circumcision does not cure or prevent any of the following: