Why is it that masturbation is considered a sin in christian theology?

The people of that time used sand as lubricant. As such their genitalia were grossly disfigured and they believed it must have been evil. They also had a habit of getting sand/semen in their eyes, leading to the belief that one would go blind.

It is because Christians misunderstand that scripture. The brothers were killed because they disobeyed God, not because they jacked off. They were supposed to knock up the sister in law by the law of those times. But, it is just a story that shows we should be obedient to God.

I guess ultimately because it's solely about self as it's not shared with another and so seen as lust.

However, Its been blown out of all proportion by religion into some great evil which strikes at the heart of the very foundation of the whole world.lol.

Whereas it should be ranked up there with forgetting to brush your teeth.

It's because the christian imaginary sky pappy told Onan to impregnate his brother's wife. Onan felt guilty about it, so he pulled out during the act, spilling his seed on the ground. Because Onan disobeyed god, god killed him. So that's why you're supposed to feel bad about playing with your stiffy.

Any Sexual activity outside of marriage between a man and a woman is a sin

Every man including all Christians have masturbated and one time. Even the pope has at one time.

It's because if you really want control over someone, you get right in there and grab hold of their sexuality... can't get much more intimate than that...

Anything fun is wrong

Anything that deals with your genitals is wrong, bad, and gross.

Because it is the spirit of LUST. That is all