How do we distinguish homosexual acts and fornication, relative to natural law?

Father Serpa-

A poster in a thread I recently started posed a question to you that asserted simply (and bluntly) that I had denied that fornication is contrary to natural law.

Lest you think I’m going around on CAF trying to legitimize a grave mortal sin, I would like to clarify the issue, and ask you for your thoughts on this issue in light of the clarification:

In an attempted to correct the erroneous assertion that homosexual acts are sinful only because they are “sex outside of marriage” I stated: “Not true. That is the reason fornication is wrong. Homosexual acts are intrinsically evil and disordered. Procreative heterosexual sex outside of marriage is sinful by virtue of its being outside of marriage. It is neither gravely disordered nor intrinsically evil, however.”

I recognize that, in my haste, I worded that very poorly and potentially misleadingly. What I meant, however (and I have since clarified this in the thread), was that, fornication, while a grave sin of the flesh, is at least a naturally ordered sexual pairing, open to the possibility of the generation of life (barring contraceptive use, of course), whereas homosexual acts are disordered and contrary to natural law at a much more fundamental, intrinsic level. My very poor wording above was an attempt to get at what St Thomas Aquinas said pertaining to this matter: “For if the sins of the flesh are commonly censurable because they lead man to that which is bestial in him, much more so is the sin against nature, by which man debases himself lower than even his animal nature.” (St. Thomas Aquinas, Super Epistulas Sancti Pauli Ad Romanum I, 26, pp. 27f)

I was trying to make the distinction between fornication and homosexual acts that St Thomas is making, and I believed it to be very significant that St Thomas reserves the phrase “sin against nature” for the act of Sodomy, and not fornication. I failed in my initial attempt to elaborate on this, so I wonder if you might assist me in this, providing me with correction where it is needed.

I greatly appreciate your help, Father.


Even though acts of fornication and homosexuality are both sins against the natural law, St. Thomas sees them as differing in degree. For him, homosexual acts are the worst type of sexual sin because they are based on the “moral species” and not on “intention or circumstances” as is fornication. Therefore he considers them to be even worse than incest, adultery, prostitution, and other sexual sins.

“Just as the ordering of right reason proceeds from man, so the order of nature is from God Himself: wherefore in sins contrary to nature, whereby the very order of nature is violated, an injury is done to God, the Author of nature. Hence Augustine says (Confess. iii, 8): ‘Those foul offenses that are against nature should be everywhere and at all times detested and punished, such as were those of the people of Sodom, which should all nations commit, they should all stand guilty of the same crime, by the law of God which hath not so made men that they should so abuse one another. For even that very intercourse which should be between God and us is violated, when that same nature, of which He is the Author, is polluted by the perversity of lust.’ “


Fr. Vincent Serpa, O.P.

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit