True. But Government was instituted because Man is not always honest and good. So Isaac’s family and fellow men around him were essentially the Government acknowledging his commitment to this woman. And if he had tried to forsake her, she could have found recourse by coming before them and stating her claim as his wife. And they would have(in theory) held Isaac to his commitment.

You can claim all day long that you are committed to this woman. But in truth you have nothing but your word to hold you bound to her. Not even her father would hold you to it, would he?