alright, so this is what i have heard. the church i go to is really into looking at the original word in greek and what it means rather than looking at the english word because it doesn't always directly translate.
in I Cor. 6:9 Paul says that the sexually immoral, among others, will not inherit the Kingdom of God. Because of the way this has been translated in english, people often believe that this just means having sex before marriage. The word in Greek is pornea (where the english word "porn" comes from). pornea means anything or any kind of sexual activity taking place outside the sanctity of marriage. this includes but is not limited to porn, strip clubs, and masturbating.
due to this, i believe masturbation is sin. do not be discouraged though, because all things are possible with God and He will not let you be tempted beyond what you can handle. He provides a way out, always!