Wasting? But it generates again afterwards, so it's not like wanking will prevent you from having children. Oh, well, those are ancient rules, so it was a different culture, and I guess it's hard to understand it in today's mentality.
Hey, I didn't say it made sense! But, spewing sperm like that is considered wasting it - if released into an egg, it could generate a child. Again, it's pretty dodgy... don't spend too much time thinking 'bout it.