As many of you may remember, I have a quite extensive background in Catholic Theology and the Catechism. So, seeing such absurdity will really bother me.
Let me make this clear - unlike Hebrew Law, the Catechism is not a series of cultural and religious dictates or norms established to protect people from health problems. As such, the prohibition against masturbation has nothing to do with sperm count or the rest.
Instead, it is a dictate on perspective that can trace its origins to Boethius, who established the necessity of perspective and view point in the relationship of sin and virtue. A sinful act is one that makes an individual animal like - they no longer think or care, they don't reason, and they no longer live for God. They instead pursue the flesh and think only of the flesh. Masturbation is not the sinful act but the manifestation of a sinful pursual of the flesh.
The Catechism requires individuals to live a chaste life because it is their exercising of the will in order to achieve a state that surpasses the animal. Animals are controlled by their passions and by their instincts. God did not make us superior to simply throw that away. Instead, free will is exercised only in chosing to do what is right. We are given the power to use our will to overcome our desires, to control our habits, and to focus on the divine instead of the worldly.
To somehow try to argue that animals doing something makes it "natural" and therefore we should allow it is absolutely absurd. We are better than animals, not worse. If anything, the argument that animals can do it would only verify the baseness of the act itself.