I have been told by the anonymous commenter that my previous response was not answered clearly and that I was perceived as "beating around the bush". Let me try again. Slavery and marriage are both greatly misunderstood in our society and both have often been linked as twin evils. They are, however, neither twins nor evil in themselves. It is our limited knowledge of God and His Word that is causing the consternation.
God's Word does approve of slavery and the marriage of men with additional wives and no amount of cultural observation can add to or take away from what He says. Our greatest difficulty arises when we try to impose modern western values, which are unstable at best to the unchangeable Word of God. He doesn't change, nor do His values. There are at least three distinct types of slavery referred to in the Bible, only one is desirable from the slaves position.
Under the Torah, when one is a prisoner of war he or she has been spared as a slave for life. A woman captured, may also become a wife to her captor, with full stature as a Hebrew wife. Men are slaves for life with little or no rights or protections, such is the fate of war. In the NT, under the Roman system, entire conquered nations are slaves to the emperor and individuals are slaves to particular masters. In both situations there is hope of adoption into Roman society or the possibility of somehow purchasing one's own freedom. The third type of slavery revealed in the Word is that of the bond-servant. In this arrangement one may be sold into servitude by parents or one may sell themselves for money or to settle a debt. This type of slavery is approved and regulated by God in the Torah. This slave has few rights and is the exclusive property of the master. He does have hope of redemption and or release at the sabbatical year. This type of slave is the one compared to the servants of God in the NT. This slavery is good and acceptable to God.
The prisoner of war and the roman style slavery is not condoned by God, but is recognized as a real state of affairs for the believer. It does not, however, transcend the liberty we have in Christ. That liberty is possible even though one is a slave in disagreeable circumstances.
The bond-servant slave is much like a person that signs an employment contract that is legally binding. While the analogy is not complete, it does give us a feel for acceptable slavery. One of my ancestors was an indentured servant. This was her way of paying for her passage to America in the late 1700's.
Whether or not our point of view supports what the Bible teaches, the Bible is always right and true and His Word always trumps human opinion. The same may be said for the Bible truth of marriage. Marriage according to God is always the relationship of one man and one woman. The same man may also have the same relationship, simultaneously with an additional wife and still be legitimate in each. God does not approve, promote or condone group marriage. What God says , He means and what He approves is always acceptable to Him, whether we like it or not. He will not change to meet our expectations. We should change to meet His.