Onan displeased God in that God did not want his people manipulating His sovereignty. Other than this sort of thing, masturbation is not found in the Bible. Reason Reason is the tool of the apologist from here on out. He will reason that sex is for marriage and procreation. He will not talk about sex being pleasurable.
Many saints really do think sex is only to make babies.
Let thy fountains be dispersed abroad, and rivers of waters in the streets. Let them be only thine own, and not strangers' with thee. The fountain is a discreet reference to the man's pleasure in copulation. The graphical language is very obvious. The sin would only be in sharing the fountain with anyone other than the god-given wife.