What is the meaning of "Ki Yasir es Bincha ... "? To whom is it referring?
Rashi: It means that the son of the Nochri who marries your daughter will take your (grand)son away from Hashem. 1
Targum Yonasan: It means that the daughters of the Nochrim entice your sons away from the service of Hashem.
Rashi (citing Kidushin, 68b): The fact that the Torah presents this case and not that of not taking his daughter for your son implies that the son of your daughter who is born from a Nochri is considered your son, but not the son of your son who is born to his daughter (a Nochris).