What is your clear and convincing evidence from the Bible that masturbation is sinful behavior?
I would ask the same of anybody else who has made comments similar to yours.
"But I say, walk by the Spirit, and you will not gratify the desires of the flesh."
"But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart."
"Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body."
"For this is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God;"
"Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry."
Some might think "I don't see the word masturbation" but the Bible doesn't always read like a book of laws, it often gives us general principles which we can apply for ourselves. It's also important to keep in mind that just because the Bible does say that something is wrong, does not necessarily mean that it isn't wrong. God gave us conscious and moral intuition for a reason; we have the ability to see immorality and recognizes it such, intuitively. This is why even atheists intuitively believe that things like murder and rape are wrong. I think that given what one knows of the world, and given what one can freely learn from the word of God, if one does not accept that masturbation is a sin that it is a result of willful ignorance,
"Now the serpent was more crafty than any of the wild animals the LORD God had made. He said to the woman, 'Did God really say that'"
I'm not sure what you believe, but that is my stance on the issue.