This sin, although rarely mentioned in name, seems to be the cause of hatred for many atheists. This ancient is actually no longer recognised by most Christian churches, and a growing number of Jewish synagogues. The reason it was introduced was not actually because of "some stupid religious thing about scaring kids into not beating off" as many active atheists may try and tell you, but a way of encouraging population growth. The new testament, the last part of the bible, was completed within a century of Jesus' death. The 'old testament', that is, the 'Jewish' section, and also used by Islam was completed before that. At this time the expected lifespan of a person was around thirty years, unless they were kings, religious leaders, upper classes etc.
This meant that people were generally having more kids to keep the population afloat. Not having children was seen as a waste of ones' seed. Masturbation was therefore discouraged by making it sinful (it harmed the population along with makinds' future). Although not strictly enforced, circumcision was the order of the day to prevent this, because it was no major crime, nor was it practically enforceable. Because fo the dropping death rate of recent centuries, especially the twentieth century where the expected life has almost doubled, it comes as no surprise no-one cares about this sin, except the anti-christian brigade of 13-year olds who 'know' that God does not exist (that is a separate matter)