Ah yes, the big enchilada of Christian singleness concerns.
I think Christians are more hung up and obessed over whether or not they masturbate than God is. I think God is more concerned with other aspects of your life than this. Having said that...
My answer:
Since the Bible does not specifically say it is not, and does not seem to even allude to it being a sin, no, it is not.
I personally believe that God gave people (including unmarried individuals) the ability to achieve sexual fulfillment on their own in this fashion in order to relieve sexual urges, if they choose to utilize this method. You may choose not to utilize it, which is fine, too.
I think God is a lot more understanding about things such as this than your average Christian.
I think God realizes it's more than a bit unrealistic to expect a 20-something / 30-something (whatever age range) unmarried person to have absolutey no sexual release what-so-ever.
The articles which argue otherwise are usually written by 50 year old married Christian pastors who have sex with their wives once every so often.
Jesus said it's wrong to think lustful thoughts, so I suppose if you engage in masturbation, as long as you do so without thinking lewd thoughts or without viewing pornography, that would be acceptable.
However, if engaging in masturabtion violates your personal conscience, if you believe it to be a sin, it would then be a sin, and I'd advise you to stop doing so.
What about Christians who disagree and think that it's inapproriate or it's sinful for Christians (or for anyone) to masturbate?
First of all, I think such Christians who are against masturbation are allowing their personal convictions color the advice they are asking all Christians to follow.
If the Bible is not clear on a moral matter (as is the case concerning masturbation), the Bible is none-the-less clear that you are to follow your personal conscience.
One well-meaning Christian says at her site (read her article here: "Sexual Purity") that if you don't think Jesus Himself would've engaged in masturbation, that you should not do so, either.
While normally I think asking one's self "What would Jesus do?" is a fine way to respond to a moral difficulty (as she seems to be doing), I cannot agree with that concept here.
I hate to even have to bring this up, but the fact is that the Bible also does not mention that Jesus sometimes urinated.
Jesus was just as much human as He was God, and the human body being what it is, well, after Jesus drank water, you know He sometimes had to relieve Himself.
Just because we find it hard or embarrassing to think that maybe Jesus had "to go" like any of the rest of us, it's a fact that He did. He certainly didn't cross his legs for His 33 years on this earth, after all.
This may not be a perfect analogy to masturbation, but while I don't think anyone likes to think about or visualize our Lord and Savior urinating, hey, we all know He did so. He had to; every human has to go.
Our own difficulty or embarrassment with the subject, or thinking of Jesus' particular handling of that subject, does not necessarily mean it's wrong or sinful to participate in that activity.
I'm not even suggesting that Jesus Himself engaged in masturbation. I have no idea if He did or not.
I'm just saying that in my view, I think it's a little "off" for one Christian to suggest to other Christians that because she herself thinks a disputable matter is a sin, that it's conclusively a sin for everyone else.
I also think it's a little "off" for one Christian to suggest that the best way to determine if masturbation is okay or not should be based upon whether they can picture Jesus doing so Himself.