okay,
anything done in excess invited trouble. sex in exess is lustful. i you do it once in a while with ur partner for pleasure, its only to sanctify thier marriage and symbolize thier love by uniting thier bodies.
same with masturbation. ppl do it to explore thier sexuality and thier bodies. so, when ur a teen it acutally pretty heathy to do so. but when you become obsessed, do with porn and crap, its in excess there fore its lustful and evil.
but masturbation itself is not initself evil, when people do it with different intentions it becomes evil.