@user6591 see the Gemara there, that this enactment was done by the beis din of Shem

"Support for such an idea might actually come from the Shulchan Aruch (Even HaEzer 16:1), which states that someone who has intercourse with a non-Jew is punished by lashes משום זונה... and is indeed very difficult to read the Shulchan Aruch as referring to the Biblical prohibition of לא תהיה קדשה." Well, the Shulchan Aruch explicitly says that if it's "derech z'nus" then the person is rabbinically liable, so it's impossible (not just difficult) to read the Shulchan Aruch this way.

@Matt I just don't understand why you suggested the S"A as a possible support for bi'ah derech z'nus being d'oraysa, when he seems to say explicitly that it's d'rabbanan.

Also, didn't you mean Hil. Issurei Bi'ah rather than Ishus? | Also, the Shulchan Aruch is ruling like the Rambam, that there is a separate category of derech ishus that actually exists, and it is d'oraysa.

@fred I'm generally averse to such definitive statements, and while I too can't think of any other way to explain the language of the shulchan aruch other than as a reference to a derabanan, perhaps I'm just not creative enough

I thought I made it clear that in the final analysis te shulchan aruch probably holds that it's derabanan

@fred but the question is about Derech znus, not Derech ishus (I thought)

@Matt You don't have to make a definitive statement beyond that which the Shulchan Aruch actually says outright, namely, "A person who has bi'ah derech ishus is biblically liable to lashes, and a person who has bi'ah derech z'nus is liable to a rabbinic flogging." This leaves the possibility open that there is a biblical prohibition for derech z'nus that carries no malkus.

@Matt "but the question is about Derech znus". Maybe. OP didn't specify, though.

@Fred (merely so as not to bog down the comments section too much), I've seen achronim explain the shulchan aruch more creatively, that there's an issur de'oraisa of kedaisha without the biblical punishment of lashes (which may be the case if there's a halakha lemoshe misinai applying this prohibition to general non-Jews)

this is how they want to be meyashev shittas rashi

@Matt OK, got it.

but it's nice to know people are reading my answers so carefully! :-)

@Matt I'm still reading it actually. :)

@Matt I think it might be a good idea to specify at the beginning of the answer that you are referring to a case of derech z'nus (though I suppose that might be gleaned from reading your answer).

@Matt (Just cleaned up my comments on your answer, as people can now find them in chat).

@Matt Also an explanation of the parameters of derech ishus vs. derech z'nus would be helpful. E.g. if a Jewish man is living with a non-Jewish girlfriend, that may count as derech ishus.

I thought that the OP was asking about Derech zenus, since the text of the question contained intercourse as opposed to marriage. And I agree that a good answer probably should contain a discussion of the exact parameters are of zenus vs. ishus but I'm not sure what it is. The rambam leaves it as kind of fuzzy in that sense I think

@fred

@Matt Derech ishus may mean that they live together (per Hil. Ishus 1:1). On the other hand, perhaps it includes any case where she is not מופקרת לכל אדם. I don't think it can mean he had intent to marry, as that's not possible.

@Matt In any case, I think the answer should at least disclaim something like: "This answer is referring only to 'derech z'nus', whatever that may be."