The physical act is no where condemned in Scripture which is the sole authority on matters of faith and practice. But it nearly always requires us to commit adultery or fornicate in our minds (hearts). That is something that is condemned by Scripture. It's extraordinarily difficult to do one without doing the other.
Truth about Masturbation
It is harmful psychologically! The mind is on self—on sex—not on a lovely wife. Invariably it produces a guilty conscience, and destroys normal confidence. Sex experience belongs in marriage! The mental attitude is the all-important factor. God intended it to be that sacred and wonderful relationship of expressing love in that most intimate and personal of all human physical and psychological experiences, which binds husband and wife together truly as one flesh! When sex is expressed in any other way, it robs one of a portion of that blissful, delightful and truly wonderful God-ordained experience! Remember love is giving—an outgoing concern—as well as sharing. Masturbation is neither.
Masturbation is either plain lust, or else a desire for relief. But God provided for relief, through the means of nocturnal emissions during sleep. If such natural relief is needed, the boy or man may induce it by sleeping on his back.
Most boys and many girls get into the habit of masturbation at an age so young they simply cannot remember its beginning after growing up. It is a nasty habit, often almost impossible to break. There is no greater plague!
I personally am undecided, but I'm just going to go ahead and click no. Nothing bad can come of it. It's not as bad as killing somebody, or doing drugs. It's in the bible, but it has a small part whereas other stuff is explained a little better. And really, is it anyone's business either way?
The original Greek word for woman in Matthew 5 where Jesus condemns lusting after a woman is very specific to mean "wife" as in "another man's wife". Therefore if you are thinking about a woman who is not another man's wife you have not violated this scripture. Also the term "lust" in both Greek and English indicates a more extreme desire, not just attraction or even just a mere fantasy. Based on this whether it is a sin or not really depends on what is in the mind and heart, not the act itself. Using it to withhold sex from a partner (as even commanded in some scriptures) will cause God to frown on it, especially if you are mad at them and refuse to work things out. However overall I see ZERO support to call it a sin in general in scripture. In hard to interpret passages the original Greek and Hebrew often gives clues as to the EXACT meaning of phrases and words.
Depending on your point of view, we were born with these parts and whether you believe we were born from nature or created by some God, we are capable of masturbating. What is created by nature/Gods can't be a sin in this case. It'd be like saying walking or press-ups are a sin.
Most occurrences of masturbation being a sin arise from the sex obsessed Catholic Church, a large number of who's priests engage in child rape.
I know I've put "no" as my answer, but as stated, I'm extremely undecided. Up until my Christmas break, I always assumed it was a sin and explained it as such without really thinking about it. But I will admit that I struggle with lust, so I became curious as to what other trusted theists think on the issue. One such theist was one of the owners of a radio program called Focus on the Family, and he wrote in an article that masturbation, in itself is not necessarily a sin. However, what I read still didn't answer the question of if it is or not. He said in the article that masturbation can become a sin if it takes place of God or other important things in your life. If you're doing it all the time, for example, it's wrong because it's taking away from time you could be doing something else. But this didn't answer my question because really anything can become a sin in that regard. Playing chess isn't bad in itself, but if you're playing chess to the extent of it taking away from time with your family, your employment, or time from God, then it can be.
I guess, as I said, I'm still very unsure. I do know that it is no where in the Bible, and I assume God would have had someone talk about it somewhere if it were a sin. He goes into much detail about lying, cheating, murder, envy, and the like, but no where is masturbation really explained. For anyone that's following my comment, please know that I actually intend to speak to one of my professors here at my college, who used to be a pastor. I am extremely curious as to what he will say and what Bible verses he will point me to, and I'm hoping to finally get some answers. This question really is tough, especially when lust is a real struggle.