God is not the source of moral law.
From early on man came to know God not merely as a bountiful provider but a God holding to definite standards in both conduct and practice. God's creative works were 'very good' why? Wasn't it because of meeting God's perfect standards in all aspects?
Without standards how could one judge with any degree of accuracy what is right or wrong? Man's accomplishments are pale in comparison with standards in nature. The universe, constellations, orbits of planets, cell structure, etc., are astounding in standardization as found in nature. Such standardization makes possible to recognize and classify plants, fish, birds, etc. in minute detail. Standardization in the human body lets physicians know where to look.
From the beginning man saw stability in day and night, gravity, etc. so doesn't it stand to reason that man also learned the moral standard for marriage from early on in what Genesis (2:4) says?
If the Creator is not the source of moral law, then who is?
Isn't God the law giver of the standard for the universe?
Man as creation then is subject to not only physical laws but capable of reasoning spiritually as well as morally. Isn't God the source of spirituality?
The physical laws can not be broken, and one that goes against or contrary to physical laws experiences immediate reactions. Likewise, moral laws can not be circumvented or violated with impunity. Although breaking natural or moral laws might not bring immediate consequences, but one reaps what one sows.
Since God is Creator of all things, then how could he not also be the source of morals?