The goodness of a thing simply is characteristic excellence. Thus a “good sandwich” is a characteristically excellent sandwich (i.e., tasty and/or nutrition), a “good dog” is a characteristically excellent dog (i.e., loyal, obedient, companionable, etc.), and a “good person” is a person who is characteristically excellent, i.e., virtuous (where “virtues” are understood as those habits of perfection that characterize the best of people – prudence, justice, temperance, fortitude, faith, hope, and charity).
So, in other words, to be a “good person” is simply to be what God made us to be. To be good is to honor the order of being which God ordained. Now it is part of human nature that he is divided into two sexes, and that these sexes exist for reproduction, hence why we call the sexual faculty the “reproductive system.” Since men have reason and free will, it follows that they are able to discern what the moral demands of his own nature and is free to act on those demands; therefore a good (i.e., characteristically excellent) man will discern the fact that he has a reproductive system which is ordered toward reproduction and will act appropriately, i.e., by using it in those manners which are appropriate to it.
(Note that my use of God above is intended to help Catholics understand the natural law. It’s not strictly speaking necessary to it. This is why natural law theology has its origins in Aristotle, who was pre-Christian).
Since, as I said, the sexual faculty is naturally ordered toward procreation, it follows that it is contrary to the natural order to use it in a contrary manner. Thus contraception, which seeks to make use of the sexual faculty while deliberately thwarting its purpose, is an evil.
Now if a thing is evil than that is harmful to the soul follows simply by definition. That’s what sin is: action contrary to one’s own good. Sin is an act of spiritual suicide, born of spiritual madness (i.e., the darkness of the intellect which elevates inferior goods above superior ones).