I think the Onan argument is not the best in the case against masturbation, and many promoters of masturbation argue that Onan was struck down for not planting his seed. He wasn’t masturbating, per se, and at the same time, in a way, he was.
Onan was having sexual relations for sexual gratification and not embracing the life-giving nature of sex. The sexual act has multiple parts- sexual pleasure and intimacy, and is life-giving.
I heard a talk by Fr. Mike Smitz online and he articulates it very well. Here’s my paraphrase of his talk:
*Tthe nature of eating is for nourishment and pleasure. If I eat for simply nourishment, I am not violating the nature of eating because the act of eating stays in tact. If I eat simply for pleasure (without overindulging), I am not violating the nature of eating because the act of eating stays in tact. However, if I eat and then deliberately make myself vomit, I am violating the act of eating because I am going against the nature and purpose of eating. It’s disordered and I harm myself.
Sex is for sexual union, intimacy, and pleasure, as well as being procreative. If a couple are wanting a child, and decide to have sex simply because the woman is ovulating, they have not gone against the nature of the sexual act… it was still unitive, pleasurable, and procreative. Likewise, if a couple have sex only for the purpose of intimacy and the woman does not happen to be fertile, they have still not violated the nature of the sexual act because it was unitive, pleasurable, and OPEN to life. *
And this is paraphrased from a talk I heard by Christopher West:
*Every organ in our body has a purpose and is complete by itself, except for our sexual organs. My heart doesn’t need another to be complete. It functions as a heart, and a liver functions fully as a liver, my spleen as a spleen… But my sexual organs make no sense by themselves. They are incomplete. The sexual organs only function FULLY when united with the opposite sex. *
Masturbation violates the nature of our sexual organs. It self stimulates them without the intention for completion of the act. It violates the unitive and procreative aspects of our sexual organs by deliberately aiming to only receive the sexual pleasure of the act. When we violate somethings nature, we do harm to it. The bulimic who throws up the first time may not feel bad afterwards. She may wonder and not see the harm in what she’s doing at first, but over time, it damages her body and her way of thinking and seeing herself. Both her physiology and psychology are damaged. Her behavior is disordered, so disordered, perhaps, that she sees nothing wrong with her behavior. Masturbation, too, may not seem bad at first, especially to the person who is masturbating, but over time, it damages the emotional and psychological well-being of the person, but even more than that, it damages their soul. Why?
God gave us the ability to procreate! This act can bring forth new life. The sexual union reflects the image of the Trinity! Masturbation defiles that image. It only seeks to receive pleasure while avoiding union and life. It draws one into themselves. When we look at the Trinity, and then look at the devil, there is an interesting parallel to the sexual/marital act v.s. masturbation. The Father is all-giving to the Son and the Son is all-giving to the Father and it brings forth a third person - The Holy Spirit. The devil, on the other hand, looks only at himself, seeks to pleasure himself, and is only concerned about himself, and therefore, only has himself.
Masturbation is pure evil because it defiles our very human nature.
