Answer

As far as I know, nobody obligates Noahides in rabbinic laws. They are not even obligated in all categories of forbidden sexual relations, so all the more so they are not obligated in the secondary rabbinic prohibitions related to forbidden relations. It seems that the explanation is that “do not deviate” does not obligate them, and therefore there is no basis to obligate them in rabbinic prohibitions.

True, they do have an obligation to obey the institutions that determine the contours of the legal system in which they are obligated by virtue of the commandment of laws, which is one of their seven commandments. But even within the legal system, the accepted approach in Jewish law is that Noahides need not operate according to our civil law code, but according to the legal system they establish for themselves. We see from this that here they are not obligated even by the Sages’ Torah-level interpretations, so all the more so not by rabbinic laws established by the Sages.

Still, this basic assumption can be discussed in light of the obligation of minors and the law of a blind person. Regarding the obligation of minors in commandments by virtue of education, Rashi and Tosafot disagree (in Berakhot and Megillah) whether this is a rabbinic obligation on the minor or a rabbinic obligation on his father to educate him (and in Kehillot Yaakov on Sukkah it is written that both apply). The question then arises: how can a minor be obligated in commandments rabbinically if the obligation to obey the rabbis derives from “do not deviate,” from which he is exempt? Is he then rabbinically obligated even in “do not deviate”? And if so, what is that itself based on? The same applies to a blind person, regarding whom there are opinions that he is obligated in commandments only rabbinically. Again, according to the views that he is exempt at the Torah level even from prohibitions (there are views that he is exempt only from positive commandments), one must ask how his obligation begins if he is not obligated in “do not deviate.” From these two examples we see that it is possible to impose rabbinic obligations on people who are not obligated in “do not deviate,” and accordingly perhaps the same would apply to gentiles.

However, perhaps this depends on the dispute between Maimonides and Nachmanides regarding the force of the Sages’ authority in their legislation, as distinct from their authority to interpret, which according to all views is based on “do not deviate.” According to Maimonides, this is based on “do not deviate,” and then what I wrote above stands. But according to Nachmanides, it is based on another source (he does not explain, and many of his commentators wondered what it is), perhaps logic. If so, then again there may be room to obligate gentiles as well, even though they are not included in “do not deviate.”
But I have thought that the basis of Nachmanides’ view is that the Sages really have no authority over us as such; rather, they speak in our name. That is, their legislation binds us because they act on our behalf, like a legislator in a state whose authority stems from the fact that he is my representative and speaks in my name. According to this, the matter is irrelevant with respect to gentiles.

Have you seen a source that argues otherwise?
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Questioner:

Thank you very much.
I haven’t seen a source that argues otherwise, but from studying Maimonides, who writes that our religious court puts people to death for violating the seven Noahide commandments, the question occurred to me: by what authority does the religious court come to coerce or punish the gentile? Since at the Torah level no obligation applies to them to obey Torah law, I began to wonder whether perhaps there are some rabbinic laws that were added onto the original obligation of the seven commandments.

You broadened the question for me and sharpened it as well.
Thank you, and have a good week—and not a burned one, God willing.

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Rabbi:
Where did you see that a Jewish religious court puts them to death? In Laws of Kings 9:14 he writes:
And how are they commanded regarding laws? They are obligated to appoint judges and magistrates in every district and every province to judge concerning these six commandments and to warn the people. And a Noahide who transgresses one of the seven commandments is executed by the sword. And for this reason all the people of Shechem were liable to death, because Shechem committed robbery and they saw and knew and did not judge him. And a Noahide is executed on the basis of one witness and one judge, without prior warning, and on the basis of relatives, but not on the testimony of a woman, and a woman may not judge for them.
This implies that it is their judges, appointed under the commandment of “laws” that is incumbent upon Noahides, who are charged with judging them concerning these six commandments.

Still, one may ask why Simeon and Levi judged the people of Shechem. Seemingly this proves that indeed their judgment is handed over to a Jewish religious court. But as we shall now see, that is not correct.

If they do not fulfill their obligation, Maimonides writes that we were commanded to see to it, as he writes in chapter 8, law 10:
Our teacher Moses gave the Torah and the commandments as an inheritance only to Israel, as it says, “an inheritance for the congregation of Jacob,” and to anyone from the other nations who wishes to convert, as it says, “like you, like the stranger.” But one who does not wish to do so is not compelled to accept Torah and commandments. So too, Moses our teacher, by the word of the Almighty, commanded us to compel all the inhabitants of the world to accept the commandments that the descendants of Noah were commanded. And anyone who does not accept them is to be executed. One who accepts them is called a resident alien everywhere, and he must accept them before three associates. And anyone who undertakes to circumcise himself and twelve months pass without his circumcising himself is considered like one of the nations.
If so, the judgment carried out by Simeon and Levi against the people of Shechem was apparently for this reason. Perhaps this is the source for Maimonides’ novelty here in law 10, that we have a duty to compel them in this matter (the Lubavitcher Rebbe made a campaign to bring people to observe the seven commandments on this basis). I do not know the source for this (it is worth checking the Frankel index on this law).

By the way, most commentators did not understand, as Maimonides did, that Simeon and Levi acted as a religious court (and they also disagreed with his reasoning that they acted lawfully). So perhaps they also disagreed with him on this point, that we have an obligation to compel them to observe their commandments. I do not think the other medieval authorities agree with him on this matter (I do not recall anyone who writes this. It is worth searching in the Frankel index on this law).

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Questioner:
Do we not say that a gentile who has relations with a married Jewish woman is stoned? And by what authority is she forbidden to him if not by the seven Noahide commandments? And who stones him?
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Rabbi:
First, the fact that he is stoned does not mean that this is carried out by a Jewish religious court. Second, if he had relations with a Jewish woman, this is what our cousins the jurists call “private international law,” meaning the law that applies when there is a dispute between citizens of two different states (not between the states themselves, in which case it is ordinary international law). In such a situation there is a gap in each side’s legal system, since the other party is not subject to it, and therefore Jewish law must establish a default rule. See Bava Kamma 37 regarding an ox belonging to a Jew that gored the ox of an idolater (do we judge according to their law or according to ours?).
By the way, according to Maimonides, the sons of Jacob judged the people of Shechem not for violating Dinah, but for failing to fulfill the commandment of laws. Had it been for violating Dinah, this would have been a case of private international law, and then there would be no proof for Maimonides’ ruling that when gentiles do not fulfill their obligations, it falls to us to deal with it (from my previous letter), because here it would be a matter between them and us. But Maimonides holds that they judged them for failing to fulfill the commandment of laws, which is an obligation incumbent on them themselves, and therefore he derived from here his novel ruling.