Well it is true that in the bible there is nothing that talks about masturbation itself, but there are plenty of scriptures regarding purity. There is this wonderful blog I read about a girl who struggled with this problem, and was having the same question. The blog is called
One Girl's Battle with Lust - By Renee Thompson (I would like to give a link, but I don't have more than 50 posts up yet.)
I just want to quote a paragraph from there which is:
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As my faith grew, I took a second look at some Bible verses I'd dismissed during my earlier search. While they didn't say not to touch, they all stressed the importance of purity. Purity, I learned, was both the choice to be pure and the action of remaining pure. In the New Testament, the Apostle Paul offered solid advice about how to choose purity: "Flee from sexual immorality" (1 Corinthians 6:18, NIV)." (In her case, she had to flee from romance novels, because they were causing the temptation.)
As a woman, I struggled with masturbation many times in my life. I learned pretty quickly after every time I gave into that temptation, I'd feel guilty, I am not happy, and I feel like God really dislikes me because of what I done. I know something is incorrect when I am trying to list all the excuses saying its okay. This makes you feel like you're on a whole roller coaster of negative emotions and thoughts, and a lot of the time it drove me away from God himself because I just couldn't face him knowing what I've done.
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While I was trying my best to be a good Christian, I overlooked the fact that being a Christian means focusing on Christ. When I spend time with God, feeling his love for me, I see lust and masturbation become the empty imitations of love that they are." - Same article
My best answer is that masturbation is a sense of empty love. Its us struggling with the flesh that can't fulfill our hearts. Our flesh always suffers from that longing for fulfillment and can never reach it, but our spirit can be filled to where we become content, and that is through Jesus Christ alone.