There's definitely two sides to this coin, some people say that masturbation is not a sin that leads to death, others say that it's a sin because Jesus commands us to deny the desires of our flesh.
So my question to you, is multi-faceted: What biblical basis for claiming that masturbation is a sin; and what is the biblical basis to claim it leads to death?
The root of the matter is lust.
Matthew 5:28 But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
I would say that the act itself is not the sin, it's the intent of the act.
This is a very difficult issue. And for someone trapped in the chains of sexual sin, there is no easy answer - but there is the stunning grace of God for freedom.
From my other answer, for context:
I would answer this question by starting with another question: Do you want to live your life seeing how close you can get to sin without actually crossing the line? Are you motivated to be the most Godly person you can be (growing daily in Christ-likeness), or are you motivated to "get away with" as much as possible without actually going to hell?
Masturbation is a tricky question, since the Bible says little, if anything, about it specifically, and there's no other person involved. But we can turn my aforementioned question on ourselves and arrive at "is masturbating bringing me closer to God, respecting myself as a person and expressing pure Godly love?" (We can also direct the original question toward the naked object of our lust; I never did meet anyone who masturbated more than very occasionally without external "assistance" of a pornographic nature either in their head or their other hand). If we answer that honestly, the conclusion seems apparent.
In my experience, people engage in masturbation far more for emotional reasons and self-esteem issues than for physical needs. The need for affection can be met healthily in many other ways than this. The myriad of sayings and research about the benefits of physical contact (like hugs) did not originate in a vacuum.
FWIW, the teaching by which God brought me freedom after many years of chains was The Theology of the Body from John Paul II* for which Christopher West has an introductory book and a CD series which may be downloaded in MP3 format at no cost.
[*] Full disclosure: I am not a Catholic, rather a Pentecostal protestant. But I have a tremendous respect for the Catholic church and her people and consider them to be fellow believers. Experiencing a tremendous victory in my life via JP II's teaching contributes greatly to this.
Pacerier asked: "When it comes to God's view of sin, white is white and black is black. There's no gray areas. What I'm trying to say is that there's no such thing as being close to crossing the line. You either cross or you don't."
It's not that simple. Based on what the bible says, engaging in sexual intercourse when unmarried seems to me to be clearly a sin, while kissing one's girlfriend seems equally clearly not, although refraining from kissing until marriage may be even better. Somewhere in between there is the "line". Is masturbation a sin? Some would argue "yes", some "no", and some "it depends on the heart attitude and motivation".
My point is that regardless of the answer, it is far better to desire to be like Christ, and have an attitude that avoids "all appearance of evil". The question becomes, not "what action is permissible", but rather "what action is best".
As St. Paul said, "not all things are beneficial":
12 “I have the right to do anything,” you say — but not everything is beneficial. “I have the right to do anything” — but I will not be mastered by anything. 13 You say, “Food for the stomach and the stomach for food, and God will destroy them both.” The body, however, is not meant for sexual immorality but for the Lord, and the Lord for the body. 14 By his power God raised the Lord from the dead, and he will raise us also. 15 Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! 16 Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, “The two will become one flesh.” 17 But whoever is united with the Lord is one with him in spirit.
18 Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. 19 Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; 20 you were bought at a price. Therefore honor God with your bodies.
1 Cor 6:12-18 (NIV) (emphasis mine)
For myself, personally, being free and remaining far away for the line of sexual immorality seems better than seeking to get as close to that line as I can. Thus for me, having received real victory over masturbation such that it's simply a non-issue in my life, I have no desire to return to that bondage, even if it could be somehow done without sinning. Other people may come to a different place, and I don't have a problem with that.
For completeness, here's the Catechism of the Catholic church's take on masturbation:
By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."137
"The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose." For here sexual pleasure is sought outside of "the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved."138 To form an equitable judgement about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.
Suffice it to say, once you know it is a sin, it is a sin. There's no aspect of the act itself that mitigates its inherent sinfulness. The only mitigating factors are wrapped up in the state of the individual.
Specifically, inside marriage you can't do it:
There is a very good book for married, and pre-marrital couples called Sheet Music. It's aimed at the less orthodox Christians, and it has some interesting things to state about masturbation. Unfortunately I don't have it with me (I'm at work) so I will have to paraphrase, and you'll have to excuse the lack of biblical references.
Basically it states that masturbation itself is just a physical activity, no more sinful than brushing your hair or squeezing a pimple. What is sinful is what happens in your mind whilst you masturbate. If you could masturbate without thinking lustful thoughts (approach it as just a physical release, like squeezing a big pimple), then that's OK. However, it's virtually impossible to masturbate without lustful thoughts.
When it comes to masturbation, there are two types of guys: those who do it, and those who lie about it.
People errantly use passages of Scripture such as Genesis 38:7-10 and many others to say that it's sin. The simple fact is that the Bible doesn't say "masturbation is a sin". Jesus said that to look on a woman with lust in your heart is the same as committing adultery.
So, according to Jesus, thinking about it is equal with the act itself. Is it wrong to lust after your wife? No. The physical act of masturbation is no sin, rather the state of the heart and mind are the determination of whether you are sinning or not.
Dr. James Dobson I think put it best:
"It is my opinion that masturbation is not much of an issue with God. It is a normal part of adolescence which involves no one else. It does not cause disease. It does not produce babies, and Jesus did not mention it in the Bible. I'm not telling you to masturbate, and I hope you won't feel the need for it. But if you do, it is my opinion that you should not struggle with guilt over it. Why do I tell you this? Because I deal with so many Christian young people who are torn apart with guilt over masturbation; they want to stop and just can't. I would like to help you avoid that agony." -- Dr. James Dobson, "Preparing for Adolescence - Straight talk to teens and parents," Regal (1979).
As Corey said there is the matter of lust. Also see:
2nd Timothy 3 (KJV)
This know also, that in the last days perilous times shall come. For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, without natural affection, trucebreakers, false accusers, incontinent, fierce, despisers of those that are good, traitors, heady, highminded, lovers of pleasures more than lovers of God
Any of those I italicized could apply.
As far as leading to death, all sin leads to spiritual death, which is separation from God. (Probably best to handle that in a separate question.)
ἁμαρτία, ας, ἡ sin; (1) of an act, a departure from doing what is right, equivalent to ἁμάρτημα sin, wrongdoing (1J 5.17); (2) as the moral consequence of having done something wrong sin, guilt (AC 3.19; 1J 1.7); (3) as the nature of wrongdoing viewed as the rejection of God by self-assertive human beings sin, evil (RO 5.12, 13; cf. 1.21); (4) especially in Johannine usage as a moral condition of human beings in revolt against God sin, being evil, sinfulness (JN 9.34; 15.24); (5) especially in Pauline usage as an abstract moral principle or force personified as evil in character sin, evil (RO 6.12); (6) especially in Hebrews as a deceiving power personified as leading human beings to guilt and destruction (HE 3.13; 12.1)
Friberg, Timothy ; Friberg, Barbara ; Miller, Neva F.: Analytical Lexicon of the Greek New Testament. Grand Rapids, Mich. : Baker Books, 2000 (Baker's Greek New Testament Library 4), S. 45
"What is right" is defined within the Bible by God; God made sex for married couples.
Many will point out that Matthew 5:28 talks about adultery - that is, a sexual relationship outside of marriage. However, for the unmarried the principle behind this verse is the same - it shows that someone that looks at a woman lustfully commits fornication. Using your imagination to produce sexual images is a step toward that which God prohibits.
