It doesn't anywhere. In fact, the Bible is eerily silent on the matter. What it DOES say is that simply thinking lustful thoughts of a woman is sinful, but not the act of masturbating itself. I think this is because it's really fine if you masturbate to your wife if she isn't wanting sex. This helps keep you from adultery and betrayal which is terrible. The Bible actually does say that a woman should not keep herself from her husband and vice versa except for mutual consent because it is so easy for men to fall into adultery if the sexual tension is not released. I could give you all of these verses if you want.