Hi. The word "porneia" is translated "sexual immorality" in Christian Bibles but is not actually a reference to any sexual act. In Hebrews 12:16, Esau is called a 'pornos' for selling his birthright. It's some kind of religious violation akin to idolatry.

And masturbation is not outlawed in Jewish law. There are actually no references in the Bible to the practice. There is no case it's somehow "bad" in God's eyes to touch one's body.

https://medium.com/belover/christianitys-porneia-problem-5df11c1e1db?sk=787059964f1b9ca6e0a591bdec607107