There is no LIST of sins. It is either an offense or it is not. Does the person who is doing it believe it is a sin? Is it their intention to be reckless and disrespectful of decency and do it whether it is prohibited or not? Alot has to do with intent and conscience. If they do not know any better (e.g., a child), then they have not violated any code of morality, law, honor or 'good behavior'.

Adam and Eve ate of the tree of conscience, and since they were the earthly parents of the human race, we inherited the ability to learn right from wrong as we grow up. There may even be some ethnic cultures that have not always considered it wrong and have allowed the practice because it was not recognized as wrong, but in modern civilized society, it is considered to 'offend decency' because it is antisocial and disrespectful towards others.

If the person who masturbates is aware that it 'offends decency' and disrespects the other person, then they are sinning against the other person. Since God is the owner of the human race, a sin against a person is a sin against God also.

Do not forsake conscience and confine your behavior to a 'code' of law or morality, develop 'personal' ethics and conscience (we have to be able to stand before the Lord with a clear conscience).