נשג"א - גזרו עליהן משום נדה דרבנן ואע"ג דעובדת כוכבים אין דמה מטמאה מן התורה שהרי היא כבהמה חכמים גזרו עליה ומשום שפחה משום עובדת כוכבים כאילו מתחתן בה דרך אישות ומשום אשת איש דבעולת בעל יש להן לבני נח דכתיב והיא בעולת בעל וישראל הרגיל בכך הואיל ובא עליה בנדתה אתי למיבעל נמי נדה ישראלית ואתי למינסב שפחה ועבר משום לא יהיה קדש ואתי למנסבה לעובדת כוכבי' ועבר אלא תתחתן ואתי למיבעל אשת איש ישראל: “נשג”א” – The sages decreed impurity regarding them (Gentile women) for the matter of niddah (ritual impurity related to menstruation) on a rabbinic level. And even though a Gentile woman’s blood does not cause impurity according to Torah law, since she is compared to an animal, the sages decreed it.
They also decreed impurity for a maidservant (shifchah) due to her status as a Gentile woman, as if one were to enter into a marital relationship with her.
And concerning a married woman (eshet ish), Gentiles do have the concept of being considered a “married woman” (be’ulat ba’al), as it is written, “and she is [another man’s] wife” (Genesis 20:3).
And regarding an Israelite who is accustomed to engaging with them: since he would be with her while she is in a state of niddah, he may come to have relations with a Jewish woman while she is in niddah.
Furthermore, he may come to marry a maidservant, thereby violating the prohibition of “There shall not be a harlot among the daughters of Israel” (Deuteronomy 23:18).
He may also come to marry a Gentile woman, violating the prohibition of “You shall not intermarry with them” (Deuteronomy 7:3).
And he may come to have relations with a married Jewish woman (eshet ish Yisrael).