In "Happy Endings," Margaret Atwood presents a number of possible plots for a typical short story about marriage. Section "A." is the most typical, uncomplicated, and therefore the most unrealistic scenario that of course results in a happy ending. The subsequent differing scenarios become more complicated but they essentially end the same way: with "A" but "under different names." This implies that a marriage is the "go-to" happy ending.
With these scenarios, Atwood is beginning a deconstruction of plots, marriage, and gender roles. The form of this work is a blend of short story and short story criticism; thus, it is a critical, self-reflexive work. This work, via the narrator, draws attention to its own form and themes in order to encourage the reader to criticize and deconstruct those forms and themes.
Atwood uses a third person narrator to be omniscient and a second person narrator to be self-referential (or self-reflexive). Atwood uses a second person narrator to address the reader ("you"), thus breaking the figurative fourth wall by directly addressing the reader and encouraging the reader to consider the cliches and stereotypes of typical short stories which always seem to have happy endings.
In "F." the narrator (second person) addresses the reader and criticizes the previous hypothetical plots, saying they might be too bourgeois.
You'll have to face it, the endings are the same however you slice it. Don't be deluded by any other endings, they're all fake, either deliberately fake, with malicious intent to deceive, or just motivated by excessive optimism if not by downright sentimentality.
Atwood (the second person narrator) began with a deconstruction of form and theme and ends with a challenge to the reader to take it further (In a sense, "Happy Endings" has no ending because it becomes the reader's project to continue): "Now try How and Why."
Answer "how" and "why" stories and relationships are structured in these ways. Note how stories are structured in certain ways with certain themes. Consider how these structures and themes reinforce things like gender roles and social expectations of marriage and the glorified sentimentality of relationships. Why would these structures (forms) and themes be repeated if they are cliche and/or downright boring? Why is marriage and material success so often used as a happy ending? And is this always a healthy maxim or trope?
These questions emerge from Atwood's story to criticize simplistic, romanticized writing but also to criticize the social expectations about gender and marriage which help sustain and glorify these unrealistic (and often misogynistic) roles and relationships. So, the form of the story (fiction/criticism) implores a formal (romantic story structure) and thematic (i.e., gender roles) critique.