Masturbation - Is it a sin?

Maybe one place to start would be in the places where the Bible does speak clearly about various sexual behaviors, as for example, in Lev. 15:16-18, 22:4, some indications may be found there.
 
And as C.S. Lewis put it, the main problem with masturbation is that it is trying to take something that was intended for giving and make it a way to try to get, instead. Even the advertisement is false, the promise of fulfillment isn't fulfilled, what is promised is not delivered, deeds whose end is like this cannot be of God, for when God promises, he then fulfills (Num. 23:19).
 
Though in this way did we just forbid scratching when we itch, or soaking in a hot tub? This verse may have a bearing in such cases:
 
"All things are lawful for me, but not all things are profitable. All things are lawful for me, but I will not be mastered by anything." (1 Co 6:12)
 
Yet Paul goes on to say (in one of the most remarkable verses in Scripture):
 
"Yet the body is not for immorality, but for the Lord, and the Lord is for the body." (1 Co 6:13)
 
So being self-seeking is a sin, we read (Rom. 2:8), and throughout a believer's life, there is a change in attitude--from selfish motives to a motive to give, for "it is more blessed to give than to receive" (Acts 20:35).