Why is contraception wrong?
Here’s the short answer: contraception is wrong because it involves using the sexual faculty while actively frustrating the ends for which it exists.
In general, the good for human beings is determined by the ends inherent in human nature. So, for example, humans are ordered toward the end of self-preservation. Therefore, it’s good for humans to preserve their own existence. Humans are ordered to know the truth; thus it’s good to have knowledge of the truth. Humans are ordered toward peaceful living in society. Therefore, it’s good to avoid harming each other unnecessarily.
From this it follows that what is good for us in the sexual arena is the achievement of the ends of our sexual faculties. What is bad for us is the use of these faculties in a way that actively frustrates their ends.
Since the ends that nature ordains our sexual faculties toward are procreation and unitive love, what is good for us with regard to our sexual faculties is to use them only in a way that is consistent with these ends. This necessarily follows from the metaphysics of the good sketched above. To use our sexual powers contrary to their natural ends (e.g., masturbation, contraception, sexual activity among members of the same sex, pornography, fornication, adultery, etc.) is no more good for us than it is good for an oak tree to fail to sink its roots deep into the ground and take in nutrients.
And since it belongs to human beings as moral agents to pursue the good and avoid evil, it is immoral (morally wrong) to use any form of contraception.