What matters most about defining marriage is what God says. If God exists, then it would make sense that we should take seriously what he says about relationships and human flourishing. Thankfully, God has spoken and revealed himself supremely in the person of Jesus who is “God with us” (Matthew 1:22).
When asked about if and when divorce is permissible, Jesus points his listeners back to God’s good creation prior to the Fall. In the garden of Eden, God made both male and female, and through the act of marriage the man and woman are united in a one-flesh union (Matthew 19:4-6). They are to be united spiritually, physically, and emotionally, ultimately serving as a symbol and image of the union between Jesus Christ and the church (Ephesians 5:25-33).
As Mark Mittelberg says in this video, God’s definition of marriage is one man and one woman united for life in an exclusive relationship. This design for marriage is meant for human flourishing! God has so constructed this one-flesh union for the good of the couple, the children, and society at large.
In their book What is Marriage? Man and Woman: A Defense, the authors summarize the topic by saying, “Marriage is, of its essence, a comprehensive union: a union of will (by consent) and body (by sexual union); inherently ordered to procreation and thus the broad sharing of family life; and calling for permanent and exclusive commitment, whatever the spouses’ preferences.”1