This is exactly the kind of mentality that gets people into heresy. Just because something is not explicitly stated in the Bible, that does not mean it’s not a valid Christian concept. For instance, the Trinity.
But okay, I’ll play this game.
Sexuality is meant to be a shared expression of physical and spiritual love. The purpose of the genitilia is directed toward its opposite (in the form of one’s spouse). As a result, all sexual love must not be directed at the self, but directed outward, to one with whom the individual has a spiritual/physical/emotional union.
Sin is “missing the mark”. It is not the breaking of a law, it is failing to live up to the purpose that God has deigned. The purpose of sexual action is a pro-creative union between a man and a woman.
Masturbation is the act of manipulating that initial purpose to sustain one’s own, self-directed sexual desire. As a result, it is not longer serving its purpose but doing something else entirely. Masturbation is the act of “missing the mark”.
Here are some sources you may like to consider:
St. John Climacus, The Ladder of Divine Descent (Step 15)
Answer from the Orthodox Church of America
Answer from Fr. Laurent of Orthodox Answers
Since the Orthodox Church, and the Christian Church for the past 2000 years has never accepted the idea that “if it isn’t in the bible, it’s invalid”, you should be comfortable enough knowing that the Orthodox Church forbids it because of its sexual ethics. It’s an implied teaching.