Why does the Holy Bible teach about MDR (marriage, divorce and remarriage)?
Marriage is a covenant relationship, reflective of the greater relationship between God and His people (Eph 5:33, Rev 21:2 among many other verses from both Old and New Testaments). This is why it is taken so seriously.
The Bible says that marriage is the joining of a man and wife so that the two become one:
“Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one. Therefore what God has joined together, let man not separate.”
The husband as the head of the household and should love his wife as Christ loved the church. The wife should submit to her husband’s headship just as the church submits to Christ’s headship. Ephesians 5:22-33
In terms of divorce and remarriage:
“It has been said, ‘Anyone who divorces his wife must give her a certificate of divorce.’ But I tell you that anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery. Matthew 5:31-32
Think those two verses says it all really.
Remarriage after the death of one partner is permissible because death dissolves the marriage covenant.
A woman is bound to her husband as long as he lives. But if her husband dies, she is free to marry anyone she wishes, but he must belong to the Lord. 1 Corinthians 7:39
The last part of this verse is often neglected by Christians: “He must belong to the Lord”. I see a number of questions on here asking if it is permissible for Christians to marry non-Christians. I think that verse answers the question very clearly.