What Bible teach about marriage?
Marriage is such a ubiquitous part of every human culture that we tend to forget we didn't create the institution. It is God who created marriage, and He gave some very specific guidance in His word.
What the Bible says about marriage
The Bible uses marriage as a metaphor for the relationship between God and His people. The entire book of Hosea is a story about the unfaithfulness of Israel as allegorized by Hosea's wife—a not-quite reformed prostitute. In Ephesians 5:32, Paul applies the metaphor to the church age. John concurred in Revelation 19:7-10 where he describes the marriage supper of the Lamb.
Marriage is the ideal situation in which to raise children. Although the Holy Spirit impregnated Mary while she was engaged, God made sure Jesus was born to two loving parents. Several women in the Bible (Genesis 19:30-38; 38:12-26) went to extraordinary lengths to get pregnant, but such measures were not endorsed by Scripture.
Marriage is the only legitimate place for sex (1 Corinthians 7:2-5). The Bible is clear: marriage is the only relationship in which people are to have sex.
Marriage takes two people and joins them in purpose. God's mission to rule and subdue and fill the earth is given to all mankind. In marriage, two people work together to fulfill their part of the mission. Perhaps the best example of this is in Aquila and Priscilla. After hearing Apollos preach the limited Gospel he'd heard from John the Baptist, Aquila and Priscilla pulled him aside together and explained the full Gospel of Christ (Acts 18:24-26). It must have worked, as some of the Corinthians revered Apollos more than even Paul (1 Corinthians 3:4-7).
What the Bible doesn't say about marriage
The Bible does not say marriage is mandatory. God does not plan for everyone to marry (1 Corinthians 7:38). Jesus didn't. Jeremiah didn't (Jeremiah 16:2). Likely neither did Paul (1 Corinthians 7:7-8). God has gifted some of His followers to make extreme sacrifices in service to Him without the help or obligation of a spouse.
The purpose of marriage is not to increase a man's power, property, or influence; it's to increase God's. The Israelites were generally very good about getting married and having babies. But although large families helped support their agrarian lifestyle, they did not ensure success. God blessed those who obeyed (Deuteronomy 11:8-9). In the New Testament, God also promises blessings for obedience—heavenly rewards. The role of wives and children is not to increase a man's blessing on earth; wives and children are the blessing (Genesis 2:18; Psalm 127:3-5).
Marriage is not a replacement for a relationship with God. In New Testament times, women had very few rights. They could usually not own property, and their nearest male relative made all the legal decisions. But when Sapphira obeyed her husband in Acts 5 by lying about how much money they had donated to the church, God still judged her as an individual (verse 10). God was not lenient because Sapphira was keeping local custom to obey her husband unquestioningly. Conversely, Abigail was praised when she went behind her foolish husband's back to give proper recompense to David for his protection of their holdings (1 Samuel 25).
The Bible says that marriage is not defined or ordained by mankind (Matthew 19:5-6). The government does not define marriage, although its validation of the marriage may be necessary for civil purposes. Culture does not define marriage. Neither does sentiment. Even the couple in the relationship have no say. God ordained that the man and woman should leave their parents and become one. God joins the two into one. By the same token, mankind does not define the dissolution of marriage, and convenience and regret do not make for divorce. It is the rejection of God's standards, either through sexual sin or disobedience, that creates divorce.
What the Bible says about being married
Finally, the Bible gives some very specific instruction regarding what individuals must do so their marriage can fulfill God's purpose. Each member must deny their natural self-centered inclinations in favor of what is best for the relationship. Husbands must love with self-sacrifice (Ephesians 5:25). Wives should understand that every team has a leader, and God has given that role to the husbands (Ephesians 5:22-23). Wives should also understand that the "helper" of Genesis 2:18 is not an administrative assistant; it is a fierce warrior called to protect and defend. A marriage can't be successful unless both the wife and husband protect each other.