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Certainly the original purpose for sex was not exclusive pleasure but procreation:
Genesis 38
6And Judah took a wife for Er his firstborn, whose name was Tamar.
7And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.
8And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.
9And Onan knew that the seed should not be his; and it came to pass, when he went in unto his lest that he should give seed to his brother. brother's wife, that he spilled it on the ground, 10And the thing which he did displeased the LORD: wherefore he slew him also.
I don´t mean masturbators should be killed, but it`s certainly a sin.
Genesis 38
6And Judah took a wife for Er his firstborn, whose name was Tamar.
7And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.
8And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.
9And Onan knew that the seed should not be his; and it came to pass, when he went in unto his lest that he should give seed to his brother. brother's wife, that he spilled it on the ground, 10And the thing which he did displeased the LORD: wherefore he slew him also.
I don´t mean masturbators should be killed, but it`s certainly a sin.
This scripture is expressing God's displeasure only with Onan's unwillingness to raise up seed on his brother's behalf, and his clever disobedience. Onan's act was not masturbatory, he simply did not complete the act of sexual intercourse.
If we use this scripture to support an argument against masturbation, simply because the seed was spilled, every man would be guilty of sinning or becoming unclean each time they had an involuntary ejaculation. Correct interpretation is very important.
In addition, masturbation is not a form of sexual intercourse. I believe your argument, from the beginning, was surrounding procreative sex versus non-procreative sex (between a man and woman). Masturbation, homosexuality, and other deviations from sexual intercourse fall under the category of perversion.