Please expound on this. Thanks
I am not intending to quibble over what is contrary to "natural law" vs "sins against nature." I am just wanting to point out that a Thomistic understanding of natural law does include things like heterosɛҳuąƖ sins of the flesh etc. In a follow up post you said "Natural heterosɛҳuąƖ relations however illicit/sinful may remain in the natural law." If I understand you correctly, i.e. that fornication for instance, although illicit is still in accord with natural law, then this would not be correct. Another poster said that, premarital or extramarital sex are sins that do not, in themselves, violate natural law." This is incorrect and is not a mere quibble but really gets to the heart of natural law.
The natural law is simply man's participation in the Eternal Law whereby God gives all things their natural tendencies and ends. Man can reflect upon this and order his actions accordingly.
The first principle of practical reason is "Good is to be done and pursued and evil is to be avoided." The more determinate principles / precepts of the natural law are founded upon those inclinations / tendencies that we have in common with other substances. All substances have an inclination towards self-preservation as do we and from this we get precepts against murder etc. All animals have a tendency to reproduce and raise offspring and from this we get precepts regarding sɛҳuąƖ sins and the raising / education of offspring. Finally, man, as a rational being has an inclination to know the truth about God, live in society etc.
For St. Thomas, any discharge of semen in such a way that deliberately precludes or does harm to the possibility of the generation and education / raising of offspring is contrary to natural law. So while heterosɛҳuąƖ sins are not directly against natural law insofar as they would prohibit the possibility of generating offspring, they are against the natural law insofar as they are contrary to the education and upbringing of children which is meant to be done within a stable union between spouses.
This of course is in no way a complete explanation of natural law. Many other aspects of natural law are often misunderstood and should be pointed out such as the fact that a naturally good or bad act is not the same as a morally good or bad act. The species of a natural act considered in and of itself in accidental to the moral species of an act. It is not the mere frustration of a natural inclination as such that makes an act immoral. Sometimes we are supposed to frustrate natural inclinations e.g. inclinations to pleasure.