Well, Onan would spill his seed on the ground to not give his brother a son. It was seen as wicked and he was put to death by God. Then later on it's about if a man spills his seed he has to immediately bathe and will still be unclean until evening. These are often interpreted as masturbation being a sin.

Buddhism says to avoid it because it can develop attachment. Buddhism doesn't have "sins", but things that cause attachment should be avoided ... thus the reason most monks and nuns won't preside over a wedding.

But I tend to fall into the mindset of "the poison must be purged".