ondon summed it up nicely about intent to harm and reduce sexual pleasure.
Actually, from the link posted up thread, circumcision was originally all about sex, and discouraging masturbation.
This modern-day phenomenon has its roots in the 1800's, when sexual pleasure was considered immoral. It was also when doctors had all sorts of strange beliefs about 'vital energy', and weren't quite sure what caused diseases.
You know, the good ol' days, when magical water was a better bet than some of the treatments of medical doctors. One outmoded belief was that people start with a certain amount of energy and inevitably run out. A pre-scientific model of disease based on this idea was called 'Reflex Neurosis', which pathologized genital stimulation. It literally meant 'self-nerve overstimulation': If you touched your highly-innervated genitalia (whether sexually or not), you would drain yourself of energy, and a disease would occur in your lungs, eyes, heart, etc.
` It was also believed that men would eventually run out of sperm, and that ejaculation was injurious to the health -- and moral constitution! Many people not only shunned masturbation, but were terrified of losing "life force" through nocturnal emissions.
In order to prevent boys from having emissions, as well as erections that are part of a normal sleep cycle, some parents were conned into buying all manner of horrific devices designed to associate pain with the genitalia.
There were penis-cooling devices, contraptions with spikes on the inside, and even one that activated a phonograph player. Chastity belts were a product of this era, rather than medieval times, as is commonly believed. They were invented, along with armored night-wear, to sell to parents as a way to keep their kids from causing themselves "harm".
In Battle Creek Michigan, anti-masturbation big shots such as surgeon John Harvey Kellogg, recommended punishing both girls and boys for "self-abuse" by holding them down, kicking and screaming, and excising their most "abuse"-prone parts.
The trauma of genital mutilation, as well as the resulting loss of sensitivity, were meant to keep these adolescents from wanting to do it again, lest they make themselves sick. (This is clearly stated in Kellogg's Treatment of Self Abuse and its Effects.) A bland vegetarian diet was believed by many to curb sexual feelings, and so Kellogg also invented Corn Flakes, and provided them at his sanitorium in Battle Creek Michigan -- along with yogurt enemas and electrifying baths. (Yes, very much like in The Road to Wellville.)
Kellogg believed that all sex was harmful, claimed to have never had sex himself, and adopted 42 foster children -- who I don't envy. He would travel around the country, paying various medical societies to have a Chair of Circumcision for promoting genital mutilation as a health measure.
In girls, he preferred using carbolic acid to burn off the external clitoris. (Later forms of medicalized 'female circumcision' were not usually as extensively harmful, or even done for the same reasons.) When this was being promoted in the U.S. and some other countries, even the female circumcision rituals of foreign cultures were interpreted as being done to get rid of 'foul-smelling' smegma in females, thus ignoring their religious significance.