He takes a beating lately for oversimplifying and possibly viewing too much of creation through the lens of sexuality, but Christopher West’s “Good News About Sex & Marriage” still IMO provides the best popular level explanation of John Paul II’s Theology of the Body. TOB provides a far more complete explanation of the full nature of human sexuality than Humanae Vitae ever did.
If you are starting from scratch, get that book and read it first to get a layman’s broad view framework of just why God made humanity as sexual beings in the first place. The rest is FAR easier to understand once you get the big picture.