The sin of lust consists in seeking venereal pleasure not in accordance with right reason. This may happen in two ways. First, in respect of the matter wherein this pleasure is sought; secondly, when, whereas there is due matter, other due circumstances are not observed. Now this same matter may be discordant with right reason in two ways. First, because it is inconsistent with the end of the venereal act. On this way, as hindering the begetting of children, there is the “vice against nature,” which attaches to every venereal act from which generation cannot follow; and, as hindering the due upbringing and advancement of the child when born, there is “simple fornication,” which is the union of an unmarried man with an unmarried woman."