His conclusion was likely guided by knowledge of God’s standard for humans as expressed in Eden, that of monogamy.
وَعَلَى ٱلْأَرْجَحِ ،‏ كَانَ ٱسْتِنْتَاجُهُ مَبْنِيًّا عَلَى مَعْرِفَةِ مِقْيَاسِ ٱللهِ ٱلَّذِي أَعْلَنَهُ لِلْبَشَرِ فِي عَدْنٍ :‏ اَلزَّوَاجِ ٱلْأُحَادِيِّ .‏