It sounds ridiculous to us, but circumcision became popular in Christian countries in the 19th century because they thought masturbation led to physical and mental illness, and they thought circumcision would stop masturbation. No, I'm not making this up. Read the following quotes from medical journals if you don't believe me:

A Short History of Circumcision in the Physicians' Own Words

Since then, people have looked for different reasons to justify circumcision, but the rates have been dropping:

USA: from 90% to 56%
Canada: from 47% to 14%
UK: from 35% to about 3% (less than 1% among Christians)
Australia: 90% to 12.6%
New Zealand: 95% to below 3% (mostly Samoans and Tongans, less than 1% among whites)
South America and Europe: never above 3% (includes many of the world's most Christian countries eg Poland, Spain, Italy, Brazil)