In all honesty, since masturbation isn’t addressed directly in the Bible…I have a hard time answering this question because it can go so many ways & I’ve read up on a lot of debates on it.

Scripture does not forbid masturbation directly. But Scripture is clear that illicit sexual fantasies are forbidden, which has a significant tie to masturbation. In Matthew 5:28, Jesus says “But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.”

So I think  the core of the issue is…the thoughts that are/can be involved in this act, are they pure & pleasing to God? Our thought life should be one of purity & joy. In that case, then yes, I believe masturbation is a sin.

Philippians 4:8 “Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable - if anything is excellent or praiseworthy - think about such things.”