Both are mortal sins if done with knowledge and consent. In order for a sin to be mortal, there must be grave matter, knowledge, and consent (or, as my first grade teacher put it, it “must be a big sin, I must know it is a sin, and I choose to do it anyway.”
The
Catechism of the Catholic Church defines each of these acts as grave matter (bold emphasis added):
"2353 **Fornication **is carnal union between an unmarried man and an unmarried woman. It is
gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children. Moreover, it is a **grave scandal **when there is corruption of the young.
2370 Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, "
every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences,
proposes, whether as an end or as a means,
to render procreation impossible" is intrinsically evil:
Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. . . . The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle . . . involves in the final analysis two irreconcilable concepts of the human person and of human sexuality."
Fornication is a Mortal sin. Using a contraceptive during this act is an accessory. The base sin is the Fornication itself.
Not exactly. Both are serious sins in their own right. Even for a married couple, using a contraceptive is a serious sin.