1)
Does "Ki" give the reason?
1.
Malbim: Yes. Marrying the Nochrim's daughters is the reason why they did like their abominations.
2)
What do we learn from "v'Yad ha'Sarim... ba'Ma'al ha'Zeh Rishonah"?
1.
Rashi: They began the sin of intermarrying with the Goyim.
2.
Sha'arei Teshuvah (3:59, from Bereishis Rabah 26:5): Any breach not from the Gedolim is not a breach.
