Consider Bridgerton (Netflix). In Season 2, Anthony Bridgerton’s romance with Kate Sheffield is almost destroyed by his toxic relationship with his mother, Violet. Violet, a Wounded Mother (widowed), has parentified Anthony, making him the Viscount and head of the family. He confuses romantic passion with duty. Kate’s role is not to replace Violet, but to liberate Anthony from his obligation. She says, in essence, "I love you, but I will not compete with your mother for your soul."
This is lazy writing.
Crazy Rich Asians (2013 book / 2018 film). Eleanor Young is the gold standard of the complex Gatekeeper. She opposes Rachel not out of malice, but out of a fierce, ancestral protection of legacy. The film’s climax is not just Rachel proving her love for Nick; it is Nick finally choosing Rachel over his mother’s approval. The romantic victory is incomplete until the son individuates.
The Graduate (1967). Mrs. Robinson is a twisted version of the Gatekeeper. She doesn’t block Ben’s romance; she co-opts it. She seduces him to prevent him from falling for her daughter, Elaine. The result is a Oedipal nightmare where the mother-figure becomes the mistress, and the romantic storyline becomes an escape pod.
A powerful romantic storyline gives the heroine agency in this dynamic. She is not merely a victim of the mother-in-law or a nurse to the wounded son. She is an observer and a boundary-setter.