God does not approve of people being involved in more than one sexual relationship simultaneously. God created man and woman in his image (Genesis 1:27) and He meant for man and woman to be united in marriage (Genesis 2:24).
How God had intended marriage to be
In answer to a question from the Pharisees, Jesus quotes Genesis 2:24 to explain how God had intended marriage to be: the union of one man and one woman, living together faithfully (Matthew 19:1-9). When a man divorces his wife and marries another woman, he commits adultery. Only when there is sexual immorality, there might be an exception. At another time, Jesus says that whoever divorces his wife for another reason than sexual immorality makes her commit adultery! (Matthew 5:32).
A man did have several wives
Nowhere in the Bible do we read about one woman having more than one husband. But it was quite common at the time of the Old Testament for a man to have several wives. Abraham had two wives (Genesis 16:3), Jacob four (Genesis 30), king David a good number (2 Samuel 5:13) and his son Solomon even a thousand (1 Kings 11:3)! We do not read about God condemning these men for taking more than one wife. But we do read in each story of all the unhappiness in these families that had its cause in polygamy.
The Samaritan woman
There is one woman in the Bible who had more than a couple of men: the Samaritan woman (John 4:1-42, especially verses 16-18). But nothing in the text indicates that she had these men simultaneously; she had probably divorced five times and lived together with the sixth man without bothering to get married. Jesus does not actually tell her that this is wrong, but we know from his teaching on this subject that He certainly did not approve. It is safe to assume that the life of this woman changed drastically after she got to know Jesus as the Messiah (John 4:39).
Remarriage after divorce
The Bible makes clear that widows or widowers can remarry. Their second marriage can be just as holy and just as blessed as their first marriage was. Young widows are even encouraged to remarry and have a family (1 Timothy 5:14).
But is it also possible for a woman, or a man, to get married again after having divorced? From what Jesus says about this topic in Matthew 5:32 we know that divorce is only allowed in case the spouse has been unfaithful (and even there, Christians have different opinions on when divorce and remarriage are allowed). In other cases, you commit adultery by remarrying, and you make the person who marries you commit adultery as well: “But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery” (Matthew 5:32).
Why is Jesus so strict? Because marriage, between a man and woman, reflects the union of Christ and the church. Christ is the head of the church and the church is his bride (Ephesians 5:25-32). He will never be unfaithful to her. It is a holy union. Likewise the union between a married man and woman is holy.
A wonderful gift
Marriage is a wonderful gift of God that allows us to enjoy sex within the safe boundaries God Himself set up. And when husband and wife are faithful to each other, it is a wonderful picture of the relationship between Christ and the church.