In Short

1

God instituted marriage to be between one man and one woman.

“The man said, ‘This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for she was taken out of man.’ That is why a man leaves his father and mother and is united to his wife, and they become one flesh.”
Genesis 2:23-24

“At the beginning of creation God ‘made them male and female.’ ‘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 flesh. Therefore what God has joined together, let no one separate.”
Mark 10:6-9

2

The primary purpose of marriage is companionship.

“The LORD God said, ‘It is not good for the man to be alone. I will make a helper suitable for him.'”
Genesis 2:18

“That is why a man leaves his father and mother and is united to his wife, and they become one flesh.”
Genesis 2:24

“He who finds a wife finds what is good and receives favor from the LORD.”
Proverbs 18:22

3

The secondary purposes of marriage are bearing children and maintaining sexual purity.

“God blessed them and said to them, ‘Be fruitful and increase in number.'”
Genesis 1:28

“Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral.”
Hebrews 13:4

“But since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband. The husband should fulfill his marital duty to his wife, and likewise the wife to her husband.”
1 Corinthians 7:2-3