פירוש על עבודה זרה 72:13

The Talmud resolves the difficulty by saying that originally only intercourse was prohibited. Shammai and Hillel prohibited being alone with a non-Jewish woman, for they feared that this would lead to intercourse. Note that there are extensive rules preventing for the most part Jewish men and women from being alone in the same room.

But another tradition says that in the time of David they already decreed against seclusion. This occurred in the wake of the David and Bathsheba affair.
The Talmud answers that the prohibition of seclusion with a Jewish woman is indeed from David’s time. However, it was not yet prohibited for a Jewish man to be secluded with a non-Jewish woman. This was prohibited by Shammai and Hillel’s students.