Halakhah for Yevamot 151:17

To not marry with idolaters: That we should not marry with the nations - and Rambam, may his memory be blessed, wrote (Mishneh Torah, Laws of Forbidden Intercourse 12:1), "Not with the seven nations and with the other nations" - as it is stated (Deuteronomy 7:3), "You shall not marry with them." And the matter of marriages is that he give his daughter to [the idolater's] son or his son to his daughter, and as the verse elucidates, "you shall not give your daughter to his son, and his daughter you shall not take for your son." And all the more so one who mates with them [himself], as he himself is included in this prohibition. And we say in Tractate Avodah Zarah 36b [that it is] by way of marriages that the Torah forbade. But even though this verse of "You shall not marry them" is written specifically about the seven nations and in their being converts - and so is it understood by our Rabbis, who said in the Gemara (Yevamot 76a), "In their being converts, they have marriages, in their being gentiles, they do not have marriages" - in that which the Scripture restates [that which is already understood], "you shall not give your daughter to his son, and his daughter you shall not take for your son," it [comes to] include the seven nations and all of the other nations even in their being gentiles. But the seven nations are forbidden even in their being converts because they were the main [source] of idolatry and its first foundation, whereas the other nations are permitted by conversion. But one who has sexual relations on occasion, such as a man who has sexual relations privately with his harlot - this is only a rabbinic prohibition, and it is the prohibition of nashgaz (the Hebrew initials of the four types of women included: menstruant, maidservant, gentile, harlot) mentioned in Avodah Zarah 76a.