Contraception divorces the sexual act from its purpose and natural consequence. Opposition to contraceptives is rooted in a teleological view of reality in which actions, beings, impulses and parts all have clear purposes and aim at final goods. The sexual act has procreation as its natural end, and the pleasure associated with this activity is a means to an end - something to encourage people to do it in the first place.
One might also say, for instance, that nourishment is the final end of eating, and we derive pleasure from food for an analogous purpose.
But contraception puts the cart before the horse, so to speak. It identifies one's own pleasure as the final purpose of the act rather than its actual natural outcome.
It would be as if someone purposefully entered a "binge and purge" cycle because they liked the taste of hamburgers, but didn't want any of the nutritional value of it. In that sense, the pleasure of eating the hamburger takes precedence over its actual final end, and nature's purposes are thwarted by a hedonistic mindset.
