The Bible tells us that marriage has an ultimate purpose. There are many purposes: babies come from marriages; a society in the furtherance of the species comes from marriage. The great purpose of marriage, laid out in Ephesians 5, is to symbolize the union of Christ and the church. That is exactly why Romans 1 points out that the great example of all human sin is same sex sexuality. It is not that this is the worst sin imaginable, but it typifies or illustrates what all of us do in so many ways: shirk that which is different from us – God.The great purpose of marriage is to symbolize the union of Christ and the church.
We delight in that which is just like us, ourselves. Marriage between a man and a woman is crucial because it testifies to the great promise and provision of the gospel: we do not simply need ourselves. We are not enough. We need the promise, the life, and the provision that only someone very different, someone the gospel names as the triune God revealed as Father, Son, and Spirit, only God can provide us life and blessing.
The fundamentally heterosexual nature of marriage serves as a parable of sorts: a lived parable for the truth that we depend on someone completely different from us to receive life, blessing, communion, and fellowship. That is, of course, why marriage does not exist in the new heaven and the new earth. Marriage will have served its ultimate role: having testified to it, having been a sign and a foretaste of that communion with God.
When we have the real thing, we do not need the appetizer before the entree. The fact that marriage passes away demonstrates that great purpose in pointing to union with God in Christ Jesus.