I just found an article that seems to throw some light on this question.
Aquinas on Sexual Sins – The Dangers of Speaking Formally
Which is worse: adultery or masturbation? Rape or masturbation? Did Aquinas teach that masturbation is a greater sin than rape, because masturbation is unnatural, and rape is not? The suggestion that masturbation is ultimately a greater sin than adultery or rape is immediately repellent to us. In fact it is ridiculous, yet certain authors attribute precisely this position to Thomas Aquinas, reading him to be saying that the sin of masturbation is simply speaking, ultimately, worse than the sins of adultery and rape. Moreover, the further argument is made that the Church upheld this position for a long time, and this (false claim) is used to attack the Church’s credibility in sexual ethics. Doubt is sometimes expressed regarding various teachings or practices of the Church, e.g., regarding the Church’s affirmation that homosexual intercourse is wrong, or the restriction of priestly ordination in the Roman Rite to those who freely choose to embrace celibacy. The argument is made that if the Church “taught for a thousand years that masturbation is a worse sin than rape”, its teaching on sexual matters can’t be very sound.
This misunderstanding of the Church’s traditional teaching on sins against nature seems to be more prevalent than I realized. I hadn’t previously realized how many authors and teachers assert that not only many medieval theologians, but even Thomas Aquinas taught this. Here are some sample quotes from books discussing masturbation in Aquinas:
[In Aquinas’s view], Because sins against nature were sins against God, they were considered more serious than sins against other people, such as adultery, seduction, and rape (John F. Schumaker, Religion and Mental Health [Oxford University Press US], 1992), 76).To make his point perfectly clear, Aquinas poses a question: are not rape and adultery worse than unnatural acts, since they harm other persons, while consensual sins against nature do not? The answer is unequivocal: the four non-procreative forms of sex are worse, since–though not harmful to others–they are sins directly against God himself as the creator of nature. According to this logic, rape, which may at least lead to pregnancy, becomes a less serious sin than masturbation (Louis Crompton, Homosexuality and Civilisation, [Harvard University Press, 2006], 188).
“A practice opposed to the pattern set for us by nature” exceeds in wickedness the seduction of an innocent of the opposite sex, adultery, and rape (II-II 154:12) (Sex from Plato to Paglia, by Alan Soble [Greenwood Publishing Group, 2006], 1053).
Have any of you encountered such positions, or seen such arguments used as arguments that the Church had (and still has) an absurd position regarding sexual sins?
One unfamiliar with the scholastic manner of speaking formally about an issue, that is, addressing precisely the question at hand, might easily get this impression from Aquinas’s treatment of sexual “sins against nature.”
Here is the text itself. In the Summa Theologiae II-II, q. 154, a. 12, Aquinas says:
In each kind of thing the worst corruption is the corruption of the principle on which other things depend. Now the principles of reason are the things in accord in nature… and therefore, to act against what is determined by nature, is most serious and base. Therefore since in the sins against nature man transgress what is determined by nature in regard to sex, the sin in this matter is the gravest kind of sin. After this is incest… while by the other species of lust one transgresses only that which is determined according to right reason, but presupposing the natural principles. But it is more contrary to reason to have sex not only contrary to the good of the offspring to be born, but also with injury to another. And therefore simple fornication, which is committed without injury to another person, is the least kind of lust.
The first objection of the article argues that sins against nature are not the worst, because they are not the most contrary to charity: “The more a sin is contrary to charity the graver it is. Now adultery, seduction and rape, which are injurious to our neighbor, seem to be more contrary to the love of our neighbor, than unnatural sins, by which no other person is injured. Therefore sin against nature is not the greatest among the species of lust.” St. Thomas replies to this objection: “As the order of right reason is from man, so the order of nature is from God himself. And therefore in sins against nature, in which the very order of nature is violated, injury is done to God himself, the one who ordains nature.”
Aquinas is focusing on the sins precisely as a violation of the right use of sexuality, and abstracting from other aspects of them. As justice is a greater virtue than chastity, so injustice is a greater evil than unchastity, and thus all things considered, Aquinas would consider rape a greater evil than masturbation or contraception.
FULL ARTICLE: pathsoflove.com/blog/2009/06/aquinas-on-sexual-sins-dangers-of-speaking-formally/
Seriously though, I agree that murder is certainly more heinous than manslaughter; however, does that really matter for the victim? If each sin wounds the mystical body of Christ, how is that body less wounded by one mortal sin versus another?