“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” (CCC# 2352). Whatever the motive, solitary sex in itself contradicts the meaning of human sexuality, which is meant by God to be shared between a man and a woman in marriage.
You ask if masturbation is viewed as a mortal or venial sin. Remember, that for a sin to be mortal, three conditions must together be met. It has to be a very serious and grave matter, which is committed with full knowledge and with deliberate consent. What we are saying is that for it to be mortal sin, it would have to be done deliberately, knowing that it is not what God wishes for us and without any regard for that. In order to judge the morality of a human act, certain conditions have to be considered. The Church recognizes, for example, that in the practice of masturbation, psychological factors including adolescent immaturity, lack of psychological balance, and even ingrained habit can influence a person’s behavior, and this could lessen or even eliminate moral responsibility.
The condition that many persons claim for their innocence regarding masturbation is habit, and we certainly know how difficult habits are to break. We must keep in mind, however, that habit does not completely destroy the voluntary nature of our acts. As Christians who are going to be held accountable for our actions, we must strive to unite ourselves to the Lord and, therefore, do all we can to curb or eliminate all habits that detach us from Him. So, if a person is masturbating and knows fully that it is wrong, and does it willingly without doing anything to resist, then he or she is guilty of grave sin.
If they are in doubt about the morality of any sexual activity, a person should talk to his or her confessor, a priest. After listening to all of the circumstances and conditions surrounding an individual’s actions, he will make a judgment and give the proper guidance. Sometimes, professional help will have to be sought. But we should be careful with this because some professionals will actually encourage masturbation, and this would be wrong. God knows that we will sometimes fail, but He does expect us to do our best to live according to His ways. He knows when we have done all we can to resist sin. If we have done that and have acknowledged and confessed our sin, then we can rest in the knowledge that we have done our best, and that He will forgive us.
Source:
catholiceducation.org/en/culture/catholic-contributions/masturbation-mortal-sin.html
Also see this thread >
forums.catholic-questions.org/showthread.php?t=243471