While we’re on the subject of defining sin and the Levitical prohibitions on sex, I’d like to offer a reminder that there is no scriptural condemnation of masturbation. None. There is one Old Testament story that people sometimes twist into being about masturbation, and there’s a lot of non-specific appeals to sexual purity. Nothing else.
Sexual purity is an idea that is highly influenced by culture, and because the Bible tends to be rather vague, you can choose to define sexual purity as broadly or as narrowly as you want. You could decide that sex is restricted to marriage, and anything outside that is not pure. Some would say that masturbation with your spouse (or simply with your spouse in mind) is still totally fine and within those parameters. Others would say it doesn’t matter who you think about, so long as it keeps you faithful to your partner the rest of the time. Still others would say there’s nothing impure about it at all, and no restrictions are required.
Whether or not you feel masturbation is sinful depends entirely on you and the effect it has on your life. If it’s enjoyable or simple or routine or exciting then it’s probably fine. It’s not getting in the way of your relationship with God anymore than a box of cookies does. If it’s addictive or guilt-ridden or compulsive it may be time to step back and find out where all that is coming from. Just know that when it comes to the Bible, there ain’t no law against it yet.