What does the late Pope John Paul II mean when he calls the sexual act the “language of the body”?

Sexuality has two aspects: the procreative (bringing forth children) and the unitive (strengthening the union of the couple). Artificially separating the unitive from the procreative brings discord to a marriage, distorts the relationship between husband and wife, and ends up harming their unity as spouses.

Pope John Paul II explains this with what he calls the "language of the body." He observes that in the sexual act, man and woman implicitly give themselves totally to one another. That is what their bodies are saying, both symbolically and literally. Sexual expression, by its very nature, implies total gift of self to the other. As Mary Shivanandan says in her book Crossing the Threshold of Love, the language of the body says, “I give myself to you completely, without reservation or condition.”