Who says Masturbation is nomal?

I dont get it. I was looking up information about mastubation and health problems then came across a bunch of "no it alls" who says masturbation is normal and healthy, natural. Who are they to say that its natural? You don't see apes, monkeys, lions or any other species doing this, how can it be naturally occuring? Apparently it must cause some type of problem because so many people worry about it and question it. People know when something is right when there is no questioning to it. I.e who questions why do we take showers?