The designed purpose of marriage is to illustrate metaphorically God’s relationship to his people (as we’ll see from Ephesians 5:22-33). And just as a car’s engine is complex, and fails if it lacks a crucial part, so the engine of marriage is complex, and fails if it lacks a crucial part. A marriage cannot rightly depict God’s relationship with his people if it lacks faithfulness, or pleasure, or fruitfulness—or sex difference. The metaphor demands it.
