Emil Bodart counts four mortal sins if there is (1) adultery with a (2) consecrated person, (3) in a sacred place, (4) when using a contraceptive (February 1993). Thomas, however, has a different approach to this. While recognizing the compounding of evil as Bodart does, Thomas calls it only one mortal sin, specified now by the difference of matter. Adultery, for example, is a sin of lust which is all the more grievous because it includes injustice (cf. ST II II, 154, 1). We might add that the meaning of mortal sin is death to the supernatural life of grace within us. What is once dead is lifeless now. You don't kill a corpse three times after it is already dead. We might say that the example given by Bodart is one mortal sin raised to the fourth power of malice.
One mortal sin is bad enough. God drove Adam and Eve from His family Garden of Eden for the one mortal sin of disobedience. The original sinners found themselves outside of God's cozy family garden of Eden, with the gate closed behind them, closed to access by a guard wielding a fiery sword. Genesis gives a hint of what eternal damnation can mean: rejection by God and separation from Him in a cold and lonesome world without His warmth and presence. In Genesis, however, Adam and Eve are helped by God to confess and repent; God then "made garments of skins for the man and his wife, and clothed them," an act which signals reconciliation. So long as we live, God gives access to repentance. Habakuk, for example, prays in the typical manner of the Bible: "In your wrath, remember compassion" (3:4).
What is important in the statement of Bodart is his assertion that contraception adds malice to illicit sexual acts. Thomas would agree with that. The condom, the IUD, the Pill, all contraceptives have this in common, that they answer to the description of Thomas as being the practice of "unnatural vice ... from which generation cannot follow" (ST 11-11, 154, 11). An unnatural act by which a person seeks sexual pleasure from which human generation cannot follow is the worst among the seven species which Thomas lists as offending against the sixth commandment (cf. Ibid. 12). He enumerates among the unnatural acts masturbation, bestiality, sodomy, acts from which generation cannot follow [read contraception]; these are acts "whereby the very order of nature is violated, in which an injury is done to God, the Author of nature" (Ibid. 12).
These unnatural acts are worse even than fornication, adultery, incest, seduction, and rape, asserts Thomas. He damns unnatural acts so severely because they oppose a principle at its source: "In every genus, worst of all is the corruption of the principle on which the rest depend . . . Therefore, since by the unnatural vices man transgresses that which has been determined by nature with regard to the use of venereal actions, it follows that in this matter this sin is gravest of all. After it comes Incest ... and the other species of lust (which) imply a transgression merely of that which is determined by right reason, on the presupposition, however, of natural principles" (Ibid. 12). Thus Thomas supports Bodart on the issue that contraception adds specific malice to already sinful acts against the sixth commandment.