Well monogamy was God's design, because he said "two will be one," and every word of God will stand, it's just that culturally poligamy may be seen of as more of a sin then it actually is. But going from my own opinion that means nothing poligamy is much harder if a man has two wifes they will likely be jealous of eachother and have to struggle even more than usual wondering "does my husband love me." In the Bible we can see this conflict between Leah and Rachel, and between there children.
Monogamy is God's design? Where do you get that from? Cuz Abraham, the guy who got the message from God in the first place, was a polygamist. So were Jacob, David,Solomon (the wise) and others.
Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul; 8: And
I gave thee thy master's house, and
thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things(2 Sam. 12).
Matthew:
25:1 Then shall the kingdom of heaven be likened unto
ten virgins, which took their lamps, and went forth to meet the bridegroom.
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25:2 And five of them were wise, and five were foolish.
25:3 They that were foolish took their lamps, and took no oil with them:
25:4 But the wise took oil in their vessels with their lamps.
25:5 While the bridegroom tarried, they all slumbered and slept...
Clearly, Jesus didn't have a problem with it.