At the beginning there was no masturbation. There was only... Adam...the very first human . He was unable to pleasure himself because it didn't matter how much we wanted to create an exiting image that would make him have an erection...all he could imagine were lakes, trees and animals... he was the only one. Alone. Then God felt pity for him and gave him Eve. At the beginning the two of them were so happy. Eventually she lost interest and he started masturbating.
That is, as a fact, the origin of masturbation -and this is not a wikipedia article-. Now you know.