Does the Bible Condemn Polygamy/Polygyny?

 Yes.

Christ condemns it. His condemnation of divorce and remarriage is a condemnation of having more than one woman and a woman having more than one man. If what he says in Matthew 19, Mark 10, and Luke 16 does not make having more than one woman unlawful then not only is divorce and remarriage permitted, so is polygamy, polygyny, prostitution, lusting after virgins, pornography as long as it doesn't involve married women, promiscuity before marriage, etc. 

This is why Jesus is not just condemning divorce and remarriage but rather that a man cannot have more than one woman, and therefore, he cannot be remarried to another person while his wife lives. 

This is why the elder or deacon who is to have a mature Christian character is to be a "one woman man."

It is why Paul says that because of sexual immorality, "each man is to have his own wife" (singular) and each "woman is to have her own husband," meaning that he is not shared with any other woman in the same way that a man having his own wife means that she is not to be shared with any other man.

Christ argues that God did not press this upon people in the old covenant because men were stubborn (i.e., the mind was set on being polyamorous), but this is not what God intended. Instead, He tells us that God made them male and female and made the two become one flesh. They, the two, are now one flesh. He quotes the LXX translation here that makes explicit the implicature of the text of Genesis 2, i.e., that there are only two becoming one and are therefore one flesh that cannot be divided. Any addition to this one flesh union, Jesus says, is adultery. Hence, He expands the adultery law, which originally only involved a married woman, to any man who joins himself to another woman while he is one flesh with his wife.

The NT is very clear. What messes people up is that they see that the OT does not condemn polygamy and presents God as even using it as a means of blessing, and therefore, in their minds, God must approve of polygamy.

The problem with this sort of reasoning is that God uses lots of less than ideal practices to bless His people. This is a part of OT theology that shows that although humans are flawed, God still blesses them. There is no law against polygamy in the OT, and hence, marrying multiply wives is not seen as contrary to God's revealed righteousness. The same goes for prostitution and divorce. God blesses Judah and eventually the whole world through the Messiah that would come about eventually through Tamar's prostitution. Certainly, God will bless through what he did not explicitly condemn because He will bless His people even when explicitly condemned evil is done.

God gives Lot children through his daughters getting him drunk and sleeping with him (something that gets the death penalty in the law). God blesses the brothers of Joseph and saves them from famine through their evil act of kidnapping and selling him into slavery (something that also gets the death penalty) penalty in the law).

Instead, even in the OT, there are texts that lean toward monogamy on top of the Genesis 2 text. Job 31:31 states that God has made a covenant with his eyes to not look upon a virgin in lust. This is likely what Christ is alluding to in Matthew 5, i.e., a married man lusting after any other woman, not just looking with lust at a married woman (as some polygamists will argue). Throughout Genesis and the OT, polygamy is presented in literary texts as hindrances to preserving God's covenant children. Likewise, Malachi 2 indicates that God desires the foreign women they divorced their wives to marry is an evil, but one might ask why they divorced their wives to marry another woman if they could have just multiplied wives. Now, of course, there may be many reasons for it, but all of these texts were used by Second Temple Judaism to argue for monogamy and God uses this to bring them into a place where they can finally be told that man cannot have more than one woman while both are living due to the one flesh union created by the two being put together by God in marriage. 

Hence, even in the OT, polygamy is not only not commanded, it is viewed as a bad thing even if it is not viewed as doing evil in the OT.

I could also argue that if the one flesh union is the way God sees marriage, and Lesbianism is evil, then to join oneself to a man who is one with a wife is to also join with the wife. This is the logic of the Levitical laws concerning incest in Chapter 18. One cannot join himself to his father's wife, not just because it is adultery but because it uncovers his father's nakedness, i.e., it is to sleep with one's father through the one flesh union. 

I could also argue that polygamy conveys a false message about God and His people/Christ and the Church. God is not married to many people groups. It's Israel alone. Christ is not married to many wives. It's the Church/Israel alone. God has a covenant with one nation, not many. One wife, not many. If our marriages are to reflect Christ's commitment to the Church then polygamy/polygyny conveys Christ's interests divided and having multiple groups that are saved apart from the Church. This is a false gospel.

Polygamy is adultery. Polygamy is sexual immorality. Polygamy is a false gospel. Does the Bible condemn polygamy/polygyny? 

Yes.

The Bible is the entire context of all 66 books. It is the sum total of the testimony of God. The Word of the Lord is not confirmed and finalized until He is done speaking. Hence, to take the OT and ignore the context of the NT is to cut God off while He is in mid-sentence. God has now made clear through Christ that what He intended in the beginning, although not observed by Israel to show their stubbornness, is to now be observed by all who follow Christ.