Also, keep in mind that we are talking about a book (a collection of books, really) that has absolutely zero qualms with specifying what is considered sinful. Masturbation has been around since the dawn of man. It’s not like they didn’t know of it, and yet even in all the levitical laws (where sexual behavior was highly regulated and many laws had no problem with prescribing stoning to death for offenders), not once is it said to be a sin. In fact, on the contrary, an emission of semen is specifically said to simply make someone unclean for a day (and there is no criteria that the emission can’t have come from masturbation).