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.