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Summa Theologica II-II q. 154 a. 2:
Nor does it matter if a man having knowledge of a woman by fornication, make sufficient provision for the upbringing of the child: because a matter that comes under the determination of the law is judged according to what happens in general, and not according to what may happen in a particular case.
In a section of 50 Questions for the Natural Law, Dr. Charles Rice references the Summa Theologica I-II q. 94 a. 4 ("Whether the natural law is the same in all men"?), regarding things that are known in natural law as true but injurious in particular cases.
An example Dr. Rice gives is restoring property to a rightful owner who would use it to fight against your country.
The fundamental concept of the Natural Law is that certain moral precepts are true whether one sees it, believes it, accepts or not. St. Thomas says that some people may not realize that all the angles of a triangle add up to 180 degrees, but it does nonetheless. People who don't know that are not well formed in math. People may not know that fornication is wrong. In the same way, those people are not well formed in conscience.
It is conscience which is the "nearest norm we have for knowing what is right and wrong, and the criterion by which God will judge the human soul" (Dr. Rice, quoting Fr. John Hardon paraphrasing the Summa), so the rightness or wrongness of a certain act is not relative objectively in the Natural Law (this is the general case). But the test of individual culpability is whether one followed ones conscience (this is the particular case).
Because
the speculative reason is busied chiefly with the necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the truth without fail.
The practical reason, on the other hand, is busied with contingent matters, about which human actions are concerned: and consequently, although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects.
Accordingly then in speculative matters truth is the same in all men, both as to principles and as to conclusions: although the truth is not known to all as regards the conclusions, but only as regards the principles which are called common notions. But in matters of action, truth or practical rectitude is not the same for all, as to matters of detail, but only as to the general principles: and where there is the same rectitude in matters of detail, it is not equally known to all.
An example showing how speculative reason can fail in a particular case:
it is right and true for all to act according to reason: and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases (ut in pluribus): but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one's country. And this principle will be found to fail the more, according as we descend further into detail, e.g. if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore.
—St. Thomas Aquinas, Summa Theologica I-II q. 94 a. 4 ("Whether the natural law is the same in all men"?)
