According to Wikipedia, Biblical scholars essentially agree with you, to the point

Bible scholars even maintain that the Bible does not claim that masturbation would be sinful.

which is pretty cool especially given my prior belief that most people agreed it was about lust. Wikipedia does also say that some Christian denominations have interpreted the sin to be as lust, though.

And Catholicism, at least, still doesn't like the ejaculation without procreation:

Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious.

Small wonder, therefore, if Holy Writ bears witness that the Divine Majesty regards with greatest detestation this horrible crime and at times has punished it with death. As St. Augustine notes, "Intercourse even with one's legitimate wife is unlawful and wicked where the conception of the offspring is prevented. Onan, the son of Juda, did this and the Lord killed him for it."

(number 54 and 55 in a Pope Pius XI encyclical)

(I never thought I'd be discussing religion on programming.dev lol)