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?" discusses someone who married off his daughter to a Nochri. The Torah did not say "Ki Tasir", since your grandson through a Nochris is not considered your son. Perhaps he is your son; the Torah is concerned for both, only it writes in the masculine?
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 - refer to 15:18:1:1.) Or, it is like a Metzi'ah that the slave found; the master receives it. Also, the children come from the master's property (his Shifchah). This does not prove that we follow the mother!
