What is the meaning of "Ki Yasir es Bincha ... "? To whom does it refer?
Rashi: It means that the son of the Cana'ani 1 who marries your daughter will cause your (grand)son to stray from Hashem. 2
Targum Yonasan: It means that the daughters of the Nochrim will lead your sons astray from the service of Hashem. 3
Rosh (based on Kidushin 68b): Your son's Nochri father-in-law will veer your son away from Hashem. The Torah is concerned for your son, but not for your grandson, since your grandson through a Nochris is not considered your son. 4
Kidushin, 48b: Although the Torah is discussing the seven nations of Cana'an, the La'av is equally applicable to all Nochrim - since the reason of leading astray your son or daughter applies across the board. 5
Rosh: It is unreasonable to confine this to the earlier case in the previous Pasuk and not to the latter one - marrying one's son to a Nochris!
Rashi (citing Kidushin, 68b): The fact that the Torah presents this case and not that of not taking a Nochri's daughter for your son implies that the son of your daughter who is born from a Nochri is considered your son, but the son of your son who is born to a Nochri's daughter is not - since he is a Nochri.
See Peirush Yonasan, who elaboratees at length.
See Torah Temnimah, note 10.
See Torah Temimah, note 9.
QUESTIONS ON RASHI
Rashi writes that "Ki Yasir
Moshav Zekenim: Had it written 'Tasir', discussing the previous case (marrying your son to a Nochris), it would include also the case before that (marrying your daughter to a Nochri). Therefore it writes "Ki Yasir", to preclude the last case, referring only to one who married his daughter to a Nochri. 1
Moshav Zekenim: Do not say that had it said Tasir, we would say that it discusses your daughter-in-law veering your son, but we cannot infer about grandchildren. If so, it would have said Tasiro; [Tasir] Bincha would be extra to teach that it refers to your grandson (he is Jewish).
Rashi writes that the Torah taught only that your grandson through a Nochri is considered your son, but through a Nochris, he is not your son. We should say that lineage normally follows the father, so all the more so, through [your son and] a Nochris is concerned your son!
Moshav Zekenim: Lineage does not follow the father when one of them is a Nochri. We find that "ha'Ishah vi'Yladeha Tihyeh la'Adoneha" (Sh'mos 21:4; a Shifchah's sons from an Eved Ivri are slaves). 1
Moshav Zekenim: This is difficult. Slaves are different, for the children are like the slave's labor! (Fathering children is considered working at night
