Kurtbethel,
In fact, the best that centuries of church scholars have come up with to criticize the matter was in Genesis when Onan was struck dead for spilling his seed on the ground and not impregnating his dead brother's wife. Even if he masturbated to produce this emission, the issue is providing some offspring, and that was what he was punished for.
The sin though, was having sex with his brother's widow to impregnate her (as he was obligated to do, per the Torah), but then withdrawing before ejaculating ("spilling his seed", a practice which the rappers today refer to as 'skeet shooting': hmmm, I dunno why?). So he was able to enjoy having sex with her, without producing offspring so his dead brother's name could live on (in our goofy ideas about genetics, we actually thought someone's genes could be passed on in this manner.... The things us guys can dream up, and women will believe! Besides, it's not like the God who created mankind was our God YHWH, and He could've given us laws that actually made any real sense!).
So with this background, the WT spins a bunch of guilt around it with absolutely no scripture to support their position. So one could wonder why this is. When you consider it is pretty much a universal practice, then they cunningly know that they can induce guilt in someone doing it, and they can also induce a portion of the "guilty" to confess this sin breach of WT dictates and then have them in a place where they can be manipulated and controlled.
As I mentioned before, the Hebrews considered male emissions as rendering one ritually unclean for a time, as did a women's menstruation. A man having sex with a menstruating women was the WORST of all possible sins, on the par with murder and idolatry. This violation could NOT be atoned by sacrifice, and was considered to bring a curse on the land, not just on the offending couple but to all inhabitants of the Land of Israel.
(interesting that JWs now state that sex during menstruation is a "matter of conscience" for the married couple. Would WTBTS ALSO state that murder and idolatry are "matters of conscience", too? NO? Why not? You downgraded the sin of sex during menstruation, a law YHWH deemed equally important!)
Some secondary damage could come from people who try to avoid this normal outlet to sexual activity, so they marry young and inappropriately, with all the problems that result from this. The Watchtower creates a problem that does not previously exist, and their is no need for it to exist other than making people fearful and subserviant to them.
Yup, purely a control issue. Nothing more, nothing less: an excuse to exercise control over others.