Pope Saint John Paul II: “Contraception is to be judged objectively so profoundly illicit that it can never, for any reason, be justified. To think, or to say, anything to the contrary is tantamount to saying that in human life there can be situations where it is legitimate not to recognize God as God. Users of contraception attribute to themselves a power that belongs only to God, the power to decide in the final instance the coming into existence of a human being.” (Address on Responsible Procreation)
Some human acts are intrinsically disordered. Such acts can never be licitly chosen because the choice of the concrete act implies the choice of its morally-disordered nature. Contraception, abortifacient contraception, and abortion are among such acts.
Intrinsically evil acts are ordered toward an evil moral object, which is the end, in terms of morality, toward which the knowingly chosen concrete act is inherently directed. The object is independent of the intended end or purpose of the act and independent of the circumstances and consequences (effects) of the act.
Intrinsically evil acts are said by the Magisterium to be “intended” or “deliberate” or “voluntary” solely because the disordered act is intentionally (deliberately, voluntarily) chosen – not because of the intended end or the consequences of the act.
Acts of contraception are ordered toward the deprivation of the procreative meaning from sexual acts. Even if a couple is infertile, each sexual act must be ordered toward the marital, unitive, and procreative meanings in the object of the act in order to be moral.
Condoms cannot morally be used to prevent the transmission of disease, regardless of marital state and even in infertile couples, because contraception is intrinsically evil and always gravely disordered. The intended end, to protect a life from harm, and the consequence that birth defects are averted, cannot justify the intentional choice (for any reason) of an inherently disordered act.