As you stated, masterbation is a primitive process existant from the creation of man, and I do not believe it is mentioned in the Bible at all, other than a vague reference to not allowing your seed to fall idle on the ground. As you noted, in the Old Testament are a great many explicit and detailed "laws" dealing with hygene and bodily functions, and while there the Bible speaks of a woman being "unclean" following menstration, it seems to say nothing of anyone being "unclean" following masterbation.
My conclusion from this is that even in the Old Testament, and even when we were "under the law", which we no longer are, since the coming of Christ and the New Testament (We are now under "Grace"), masterbation, in and of itself, was/is NOT sinful.
The Ten Commandments clearly say nothing directly of masterbation, and indirectly only insofar as you are directed not to commit adultry. When you masterbate, while married, and you fantasize of sex with one other than your spouse, by the strict interpretation in that you would have committed adultry in your mind/heart. It is only in this, and additional fantasizing while masterbating, that I think it POSSIBLY to be sin.
I DO NOT KNOW whether it is EVER a sin to masterbate. I think it is POSSIBLE it COULD be a sin IF while you masterbate, you engage in illicit and/or "perverted" fantasizing. And again, if this were the worst "sin" God were to see in your life, you would be, in my opinion, the second most sinless man to have ever lived, second only to Jesus.
My beliefs are that FIRST, with the first coming of Jesus and the advent of Grace as expressed in the Gospel of the New Testament, the law was rendered null and void to Christians. Thus, even though under the law you would be sinful and subject to judgment, under Grace through the sacrifice of Jesus, you are not subject to judgment under the law, but judgment under Grace.
The Bible seems to say it is possible to live a perfect life, but I failed in that long ago. You see, there are two possibilities: perfect and not perfect. No grey area....perfection or non perfection. God and Jesus set that standard. By the law all non perfect would be subject to judgment, but under Grace, you are saved. Not because you earned it by having been perfect, but because God freely gave it to you by virtue of your faith.
This all said, I think, to repeat, that at worst, masterbation is a sin, if at all, only in the way and manner you think and fantasize while engaging in it. Were I to masterbate visualizing only thoughts of sex with my wife, I would challenge anyone to prove that was a sin of any nature or degree at all. The Bible says all things in a marital relationship between a man and a woman God joined in marriage are acceptable. However, I can see from one perspective that to engage in certain perverse thoughts while masterbating, over and over, going deeper and deeper into it, could eventually result in some sinful action in your life as a result of acting on such fantasies. Some have said that in such a way porn leads to acts of violence against women.
I personally feel, as I have tried to say, that there are many other things in our lives we should be more concerned about, even from the perspective of trying to live as perfect a life as God would desire us to live, than masterbation.
Be well!