If someone has a willing partner and prefers to masturbate, there IS a problem.
A book I was reading refered to a study that said that those with partners and an active sex life masturbate more frequently than those without.
I was speaking of the person who has a partner who, is waiting and willing in bed, but the other person prefers to go into the bathroom and masturbate.
The idea was that some of your soul left your body each time, or somesuch, so it was a sin similar to suicide....
I don't see how something that keeps the rape rate down can in itself be evil, but I think over doing it is socially beneficial.