What the Catholic Church Teaches

"Lust is disordered desire for or inordinate enjoyment of sexual pleasure.  Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.

By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure.  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.

The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose.  For here sexual pleasure is sought outside of the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved.

Fornication is carnal union between an unmarried man and an unmarried woman.  It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children.
Moreover, it is a grave scandal when there is corruption of the young.

Pornography consists in removing real or simulated sexual acts from the intimacy of the partners, in order to display them deliberately to third parties.
It offends against chastity because it perverts the conjugal act, the intimate giving of spouses to each other.  It does grave injury to the dignity of its participants, (actors, vendors, the public), since each one becomes an object of base pleasure and illicit profit for others. 
It immerses all who are involved in the illusion of a fantasy world. 
It is a grave offense.

Civil authorities should prevent the production and distribution of pornographic materials."

                                                              - Catechism of the Catholic Church (2351-54)