Why does the church say that watching pornography is sinful, when the watchers are consenting adults?
According to the Catechism of the Catholic Church (CCC) 2354: 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.
It appears that the Church’s teachings were ahead of modern science in recognizing the dangers of pornography, as there are also serious relational, mental and physical health issues that arise from use of pornography.
Countenance of God 