(Not Catholic, but Orthodox, which is similar enough in this matter.) The idea is more along the lines of saying that just as a human person is both body and soul, and neglect of either leads to death, the Catholic view is that the marriage itself, as well as each "marital act" should be: "Free (voluntary in the fullest sense), Total (complete sharing of self & life with the other), Faithful (to each other, exclusively), and Fruitful (pretty straightforward)". It wouldn't be good enough to say a woman married freely, so consent doesn't matter for each act. Or that a couple is generally monogamous, with some exceptions. So the "procreative" and "unitive" purposes of sex are considered to be inseparable without some debasement of the act. Hope that helps.