[quote="DavidPalm, post:35, topic:242695"]
Let me give it a try.
When we speak of natural disorder we are talking about a principle in which the very end to which a given part of the body is ordered is deliberately and wholly frustrated.
So with regard to masturbation, the primary end of procreation and even the secondary end of unitive spousal love are deliberately frustrated so that the pleasure itself may be had in isolation from those ends. Masturbating wholly negates the order of the genitals.
In the analogy I gave above, it would be disordered purposely to vomit one's food so that one may enjoy its taste and yet not derive any nutrition from it, because eating is ordered to nourishing the body. Vomiting one's food wholly negates the order of the stomach.
In these actions the disorder comes from an action which wholly, deliberately, and necessarily thwarts these ends.
Now breathing is ordered to supplying the body with oxygen. And I think the principle breaks down when applied to smoking. When a man smokes, he certainly does not do so purposely to frustrate the end of getting oxygen. Neither does smoke wholly negate the ends to which the lungs are ordered, any more than eating some rich food which is not altogether healthy for us wholly negates the order of the stomach, as long as the nutrition derived from it is received. But just as it is a sin to eat to excess and truly harm the body, so too according to the Catechism (2290) it is a sin to smoke to excess.
Just my $0.02 U.S.
Good insight. The only comment I would make is that the dual purpose of sexual intercourse, procreative and unitive are co-equal. There is no primary or secondary. That is why IVF is gravely wrong because it subverts the unitive aspect of the marital act.