Does writing an 'urfi' marriage contract without thoroughly considering its legal ruling (is it permissible or forbidden), habitually writing down information without thinking, make it permissible?

If the 'Urfi marriage fulfills the conditions of a valid Nikah (definition), then there is no harm in writing it on the Internet. However, if it does not fulfill those conditions, then the writer must make that clear to its owner. Otherwise, he must dispose of what he has written, because that would be aiding in sin and transgression.