From Kate Gladstone:

Re: Gospel of Matthew linked to bloody trail of self-mutilations (June 13): “Matthew 19:12 syndrome” (self castration because of a Bible verse) is nothing new — see http://www.transchristians.org/archive/the-practice-of-self-castration-in-early-christianity.

The reason that all the self-castrators quote Matthew (instead of the similar material in Mark) is probably quite simple — Matthew comes before Mark in the New Testament, and these are people who probably set out to read the New Testament in order and do everything it says. Therefore, they apply the knife before they even get to Mark.