I don't necessarily think it's considered a sin, but it probably isn't that great. It feeds lust and causes you to become more susceptible to adultery or fornication. It's one of those grey areas of the Bible.
That is a tough question in regard to the fact that it is seldom if ever talked about from the pulpit or in church groups. Not sure if there are Bible references to the subject. It may go under the category of lusting which would make it a sin if you had other than your spouse in mind at the time. Hmmm. Ouch!
The Bible does not specifically mention the word masturbation. However, God’s Word commands Christians: “Deaden, therefore, your body members that are upon the earth as respects fornication, uncleanness, and improper sexual appetite.”—Colossians 3:5.
Masturbation leads to unwholesome thoughts and actions such as viewing pornography and fornication. These are things that God detests.
Yes, masturbation is a sin. Our sexual organs are not playthings to be used for self-gratification. They are intended for sexual relations in marriage, to express and share mutual, self-donating love, and for the procreation of children. When we use our sexual power of loving and procreating within marriage according to God's purpose, we honor Him and our bodies. When we do not, we dishonor God and our bodies, something Scripture condemns. Masturbation is often called "self-abuse" precisely because the person who masturbates abuses his or her body, which is a temple of the Holy Spirit. Masturbation is a sin against nature - which was designed and given order by God. In masturbation, it is pleasure alone that becomes the end goal, not the joy in doing something truly loving.