Some times it appears that God has double standards. Israel must not distress the Moabites (Deut. 2:9) but God disallows them from entering into the congregation of the Lord. It does appear that getting into the congregation of the Lord requires a hire standard of righteousness but that lower standard does not earmark one for destruction. Such was the Grace of God meted out to Cain. Though he had killed his brother God put a mark on him so that no one would harm him.  

But who were these people that were so guarded? They were the children and grandchildren of Lot. Lot was their father, but his girl children for fear of never having children they drugged their father and slept with him, conceived and bore Ammon and Moab and plunged God into the regularization of the outcome of an incestuous relationship

The nakedness of thy father or the nakedness of thy mother shalt thou not uncover: she is thy mother; thou shalt not uncover her nakedness. Lev.. 18:7.   It is important to recognize that the silence does not mean sanction. It is important to remember that Ecc: 8:11
Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil  and that God gives time to repent.     

To cement the idea of the delayed mercy and goodness of God, and to wink at the ignorance of his feeble creation he chose Ruth, A Moabitess to be a foreparent of The Lord and Saviour Jesus Christ.        

The Lord said, "Distress not the Moabites for I have given Ar to the children of Lot. We don't know when this gift took place. But here again we have a covenantal transaction which cannot be changed.  Although these children were the result of an incestuous relationship, that does not change God's plan.