Mr. dB said:
That comes from the story of Onan. And I believe it is a misinterpretation, due to ignorance of Jewish law and the customs of that time. Onan's brother died childless, and it was Onan's duty to impregnate his brother's widow according to Hebrew tradition. Onan "spilled his seed" instead. The sin wasn't masturbation, it was his failure to perform his duty.
Besides, that's Old Testament. Accepting Jesus as one's savior supercedes Jewish Law, which is why Christians can eat pork and don't have to circumcise. As far as I'm concerned, if there is nothing in The New Testament forbidding masturbation, it is not forbidden and not sinful. The Ten Commandments and The Golden Rule are pretty much all the "law" that a Christian needs.
I'm not even sure that fornication is expressly forbidden either. Adultery is, but that is a very specific case of fornication. Adultery involves one or more MARRIED parties fornicating with a different partner. So unless you're boinking a married person, or cheating on your spouse, you're not committing Adultery and, therefore, not violating the Commandment.
Many people seem to confuse Victorian-era prudishness with religious law and custom. This is almost as amusing as people who think that using the archaic English of The King James Bible when they pray is somehow more "holy" than speaking in standard English.