When we look at a cracked relationship in fiction, we see two people who have survived something. Maybe they hurt each other. Maybe the world hurt them. But they are still standing. That crack is a scar, and scars are proof of life. It tells the audience: This love is hard-won.
No soft-focus montages. In cracked storylines, sex can be a weapon, a Band-Aid, a battlefield, or a eulogy. It is rarely just pleasure.
Classic examples include Shakespeare's star-crossed lovers, Romeo and Juliet, or the illicit affair between Lady Macbeth and her husband in Macbeth. More modern takes on forbidden love can be seen in TV shows like Outlander, where Claire Randall's marriage to Frank and her subsequent romance with Jamie Fraser create a deliciously complicated love triangle.