Christians must learn the spirit of the mosaic law in order to understand how God wants us to live. It is clear that he did not want polygamy to be a normal way of life and anyone who seeks his friendship would take that to heart and apply his laws in ones own life.
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Just as God originally purposed, so we must do likewise.
So, would you stone a girl to death if she had sex with someone before she had with it her husband without telling him? Let's read on a couple of chapters later...
Deuteronomy chapter 22:
"13 If any man take a wife, and go in unto her, and hate her,
14 And give occasions of speech against her, and bring up an evil name upon her, and say, I took this woman, and when I came to her, I found her not a maid:
15 Then shall the father of the damsel, and her mother, take and bring forth the tokens of the damsel's virginity unto the elders of the city in the gate:
16 And the damsel's father shall say unto the elders, I gave my daughter unto this man to wife, and he hateth her;
17 And, lo, he hath given occasions of speech against her, saying, I found not thy daughter a maid; and yet these are the tokens of my daughter's virginity. And they shall spread the cloth before the elders of the city.
18 And the elders of that city shall take that man and chastise him;
19 And they shall amerce him in an hundred shekels of silver, and give them unto the father of the damsel, because he hath brought up an evil name upon a virgin of Israel: and she shall be his wife; he may not put her away all his days.
20 But if this thing be true, and the tokens of virginity be not found for the damsel:
21 Then they shall bring out the damsel to the door of her father's house, and the men of her city shall stone her with stones that she die: because she hath wrought folly in Israel, to play the whore in her father's house: so shalt thou put evil away from among you."