I am not arguing that it is forbidden, but is there really "do not place a stumbling block" in this case, when this is a case of "one side of the river," since the prohibited thing is available to the non-Jew in any event?

First, there is also a rabbinic prohibition in a case of "one side of the river." Second, the prohibition is not available to him in every way regardless. Stimulation from one woman is not a substitute for another.

I am not sure there is such a rabbinic prohibition toward a non-Jew. In any case, regarding your second point, I saw that this seems implied by Tosafot on Avodah Zarah 14b, s.v. "Makom."

Michi,
regarding item 1 of your first response, "Modesty is not only about not causing others to stumble. First and foremost, it is for your own sake"—after all, Nadav's monumental article, "Are There Laws of Modesty?," is familiar and well known to you. If the entire area depends on psychology and social norms, as has been published, then why, at the beach, where the prevailing social norm for women is to wear a bikini, would wearing the clothing customary in that society in that place and situation be considered immodest?

See the monumental entry here: https://he.m.wikipedia.org/wiki/%D7%94%D7%92%D7%96%D7%9E%D7%94

In my humble opinion, there is a difference between a situation that society defines as modest and a situation in which it is permitted to be immodest. One would have to be very dishonest to say that a Western women's swimsuit does not emphasize areas that are usually covered, and it is simple and obvious that this raises far more improper thoughts in every healthy human male. Feel free to watch an average Hollywood movie to see how male characters relate to the "modesty" of the beach (and not only there).

Schrödinger,
your nonsense this time managed to surpass even your other nonsense. More power to you for babbling.