Obviously, the first place to start would be the Catechism, and relevant documents like Humanae Vitae, the various talks and pronouncements on the Theology of the Body, etc.
For a natural law take on human sexuality, see Edward Feser’s Aquinas or The Last Superstition. Both essentially the same book, but the former is dry and academic while the latter is a more polemical take for the philosophical layman.
The value ethics of Von Hildebrand (e.g., Christian Ethics) might also be useful here. I like Von Hildebrand because Thomist teleological ethics seem too instrumental, such that I should, for instance, help other people because doing so is good for me and will get me to Heaven. Von Hildebrand incorporated into this the Kantian insight that a man can love justice and other goods for their own sake, and that interpersonal love is a rational response to the intrinsic value of another. He’s very influential with some of the recent popes, and you can see this kind of thinking seeping through the Catechism. To put a specifically sexual spin on it, Von Hildebrand might object to contraception because it amounts to treating one’s partner as a means to the end of sexual pleasure. By contrast, unprotected sex is, by nature, an explicit commitment of the body and soul to the endeavor of familyhood with one’s partner.
