On the ball,
I wasn’t saying that histaklus = hirhur I was saying that histaklus without hirhur isn’t assur (hirhur without histaklus is).
As the Rambam writes hamistakel b’etzba ketana shel isha ??????? ?????. If there is no kavana to have pleasure then he has not violated the issur.
L’maysa there appears to be a stirah in the Rambam. In Issurei Biah (21:2) Rambam seems to hold that histaklus is d’rabanan and in hilchos Teshuva (4:4) he bring the posuk of Lo Sasuru.
M’drabanan non-Jewish women have a din Nidda. They are therefore all arayos (d’rabanan).