Another interpretation: What is the Merciful One warning him about here? This is what He is saying to him: Do not marry a gentile woman or a maidservant, so that your seed should not stray after her.
What the Merciful One Was Truly Warning Abraham Against
Yalkut Shimoni on Torah 81:10
דָּבָר אַחֵר, מַאי קָא מַזְהַר לֵיהּ רַחְמָנָא, הָכִי קָאָמַר לֵיהּ, לֹא תִּנְסוֹב גּוֹיָה וְשִׁפְחָה, דְּלֹא לֵיזִיל זַרְעֲךָ בַּתְרָהּ.