§ I. On Fornication

Fornication is copulation of an unmarried man with an unmarried woman by mutual consent; wherefore concubinage, which is the habitual commerce of an unmarried man with an unmarried woman, and prostitution, which is the state of a woman prostituted to all, are referred to fornication.

I say 1st. Fornication is always a mortal sin, because it is gravely prohibited by natural right. For if fornication were not forbidden under grave matter, men would generally withdraw from contracting marriage, and offspring received from fornication would be abandoned without any education: but this would redound to the grave harm of the whole society. Therefore. Besides, it is clear from I Cor. vi, 9; and Eph. v, 5.

I say 2nd. The circumstance of concubinage or prostitution is not necessarily to be declared in confession, since of itself it does not change the species of the sin. But by accident it must more often be explained for manifesting the state and kind of occasions, and it must always be disclosed to the confessor when he asks. - Cf. St. Alphonsus, n. 432.