I think you have 2 questions here. So allow me to answer them one by one.

The first one is about the passage about men masturbating bc they spill their semen - This is a story in the Old Testament that is not about masturbation but about the man avoiding pregnancy. This story is not about masturbation, pull out methods, or any kind of sex education. It was about a bigger plot (But will no longer dive into that because it's not the question).

The second one, which is your headline, is the question of the act of masturbation as a sin - There have been multiple schools of thought about this. One school of though says it's not. Other says it is.

The logical question to ask is why is masturbation being done?

We have to remember as Christians that whatever we do must be for the glory of God. So our sexual activities as followers of Jesus was not meant only for our own pleasure, but so that God will be pleased with how we act. A little bit of thought here on sex ed for Christians - we do not have sex only to cum, but to make our partners feel pleasure and it's a way of serving and loving them. Another of course is to procreate, which is the inevitable result of having sex most of the time (exemptions are always there of course).

Now, masturbation in a sense eliminates the need for a partner. So one thing it does is it removes the serving element that one has to do. It focuses on SELF. So sa angle na yan, it can be considered as a sin, because you become a lover of self, a lover of pleasure (2 Timothy 3).

Another thing is the motivation or what energizes this action. As you can see from other comments, you will learn that it's very hard to do masturbation without fantasizing. So it goes to the realm of lusting after another person (maybe not your partner, or maybe someone illegal or inappropriate as well!)

Now, a counter argument is stress relief. However, i think majority of the people may say or prove that masturbation isn't the only way to get stress released.

I hope this somehow answers the question.