Why is masturbation considered a sin by Catholic Church?

In spite of what medicine and psychologists have found out in recent years, masturbation continues to be regarded as a sin by Catholic Church. 

According to the World Health Organisation masturbation is absolutely normal since 97 % of male teenagers practise it. Especially for male teenagers masturbation is a fundamental step to develop a proper sexuality. The archaic point of view of Catholic Church, based on tradition rather than science, can be harmful especially for those teenagers who are less self-confident and have low self-esteem. In fact they may experience an unjust feeling of guilt which can prevent them from developing their sexuality harmoniously. This is the reason why it is important that everybody should sign this petition.