Photo ~ The Penitent Magdalen by Georges de La Tour

Human Nature and Aquinas’ Taxonomy of Sexual Sins

The following listing from Aquinas proceeds from the most serious to the least serious.

The most serious sexual sins (leaving out circumstances such as violence, which compound the sinfulness) are sins contra naturam, sins contrary to human nature, and thus contrary to God the author of human nature.

The most obviously unnatural sin is “bestiality,” i.e., sexual intercourse with animals – a sin which offers an affront to the human species. Next in seriousness is sodomy, which is an affront to the natural relationship between male and female.

In third place are unnatural coital relationships between men and women – for example, anal intercourse, coitus interruptus, or other contraceptive measures – all of which are sinful because they do not observe “the right manner of copulation.”

In his Summa contra gentiles, Aquinas compares such relationships to homicide: “After the sin of homicide whereby a human nature already in existence is destroyed, this type of sin appears to take next place, for by it the generation of human nature is precluded.”

By taking measures to prevent a human life from emerging naturally, such non-procreative sex constitutes an action against the potential human soul that might result.

The least serious “unnatural” sexual sin is masturbation, in which pleasure is intentionally sought in isolation from natural social relationships.

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