These stories speak to a lot of us who remember too vividly how awful it felt to be so awkward, out of place, and doubtful about our worth. It's also possible that they hold their power over us because we have unresolved issues about that part of our lives. We expect to achieve a vicarious resolution through these stories, and woe to the author who doesn't deliver.
That's what happened with a book I just finished. In this case the heroine's long-time buddy agrees to play the loving beau and escort the heroine home for her evil step-sister's wedding. Of course, thrown into this close proximity, sparks fly. The couple denies the attraction; they argue; nearly step over the line; back away, and then . . . Then, to my total dismay, the hero allows the truly evil step-sister to seduce him on the eve of her wedding.
I finished the book. Mostly to see if the writer could recover from this misstep. I was skeptical that it could be done. In the end, though she came close, the author failed to rescue the story. Why? Not because he had sex with someone other than the heroine (although that would be enough for some readers) but because the hero betrayed the heroine with her arch-rival. He slept with the one person who was capable of and willing to drive a stake through her heart. Even though, in his mind, it was "only sex," the magnitude of this betrayal made it unforgivable. Some things are.
Have you seen behavior in a romance that you felt was unforgivable? If not, what would qualify?