Masturbation is a sin because a person consents in their own heart to take sexual gratification from impure thoughts. It is sinful to indulge in the sexual pleasure that is derived from the satisfaction of using one's imagination to FIXATE upon obscene images, or obscene actions, or forbidden sex. Lustfully fixating upon genitals, or sexual acts, is idolatry. This is called: the worship of obscenities. This is the sin of those addicted to pornography. That's often a "guy" thing. Alternatively, taking delight in the lust of another, or consummating a fantasy of forbidden sexual romance, is the sin of adultery in the heart. That's often a "girl" thing. The act of masturbation is an outward act that confirms that one's heart and mind has consented to an inward act of impurity. Only by consenting to an imagined act of impurity can one attain orgasm. To attain sexual release, a person's will consents to LOVE something that is sexually impure.
A sin consists in loving something that God hates, or in hating something that God loves. Because God is pure love, in one sense there is no hatred in God. But there are actions that God regards with a COMPLETE ABSENCE of love. There are many things that men can do that God does not love. In so far as God does not love a human action, according to our way of thinking it can be said that God hates it. These actions are sins. They are often as hateful to men as they are to God! What are some things that God does not love? What are some things that, according to our human way of speaking, we might say that God hates??
Above all, God hates falsehood. God hates any form of lie, rejoicing in cruel lies, or in telling harmful lies about others. He hates any kind of injustice perpetrated upon his children! God hates cruelty, or the defrauding of a worker of his just wages. God hates it when one person entices another person to do something harmful to himself or others: like to worship evil, or to steal, or to lie, or to take drugs. God also hates impure sexual lust because lust is a false pretense of love. It is a gross and disordered counterfeit of what should really be a sentiment of holy love. Lust consists in fixating on the body of another human being as a sexual object - solely for sexual gratification. When a person lusts, he is making no lasting commitment to the other human being. He is regarding the other human being's body as a sexual object to be used for his own gratification - but nothing more! Outside of marriage, a person who lusts is acting as if he would take another person sexually to be his spouse: but he has absolutely no intention of actually doing so. That is why LUST IS A LIE.
When the subject of masturbation arises, it always refers to stimulating one's self for the gratification of sexual lust, or in stimulating one's self sexually while fantasizing about the lust of another person. In impure sexual fantasy, a person uses his own imagination as a pornographic tool to give his body the ability to attain sexual gratification. In reality entertaining lustful fantasy is embracing a falsehood of the heart and mind, in an act of abuse of his own body in order to attain selfish sexual gratification. This is sexual idolatry! God hates falsehood - especially worshipping a false god! This is why God hates impurity in all it's forms.
God hates the worship of ugliness, or obscenity, or bestiality. A person who might sexually gratify himself while looking at bestiality is acting as if bestiality is a good thing - when it is a hateful thing! In the same way: lustfully fixating upon lurid sexual pornography is a behavior that pretends that some obscene sexual image, or a lurid posture, or an obscene action is a good thing, when they are nothing but obscene and hateful images! These images are used as objects of false worship.