פירוש על עבודה זרה 72:6

R. Shimon b. Yohai says that the Torah already prohibits a Jew from having relations with any non-Jewish women, for any such woman could end up turning the Jew away from God. So then what did Hillel and Shammai’s students do?

The Torah prohibited marriage with a non-Jewish woman. Marriage leads to the highest level of cultural assimilation, and therefore it was prohibited. The rabbis added that even nonmarital sex is prohibited, also for fear of this leading to idol worship.