Sin is sin. God changes not.
You need to read Acts 10:11-15
Acts 10:11-15 King James Version (KJV)
11 And saw heaven opened, and a certain vessel descending upon him, as it had been a great sheet knit at the four corners, and let down to the earth:
12 Wherein were all manner of fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air.
13 And there came a voice to him, Rise, Peter; kill, and eat.
14 But Peter said, Not so, Lord; for I have never eaten any thing that is common or unclean.
15 And the voice spake unto him again the second time,
What God hath cleansed, that call not thou common.
God was trying to help Peter understand that Cornelius was not unclean anymore, because God had announced gentiles clean now and they could be members of his Church.
What made them clean? God made them acceptable to be members, and Peter obeyed and went to the house of Cornelius and baptized his whole household.
Point of the story, if God says plural marriage is clean now, who are you to get your biases all up in an uproar and fight against God. God said to all Israelites that plural marriage is clean. If God says plural marriage is clean, it is no more adultery. So your sin is sin, God changes not is not a correct statement according to Acts 10, otherwise you would not be a Christian today since you are not a Jew. God had to have changed his mind as to who could be members of his Church, which he did very effectively in Acts 10. Thank goodness, now you, a gentile can be a member.