>>283010121
This ex post facto logic does not apply. In the show Sylphie is initially shocked by Rudeus' declaration he intends to marry Roxy as well, and that he cheated on Sylphie. But once she gathers herself, she's calm, collected, and acts decisively. And there's nothing about her conduct in that scene or after that suggests she's operating out of a fear the Rudeus will leave her.
Of course as a viewer we don't know the internal states of characters, but I never got the impression she was anything less than forgiving and graceful in this scene. That's why I consider it one of her finest moments. And if we steelman that argument and say, ok, she was afraid, she was terrified: then this shows her bravery in the face of that fear, her willingness to not just overlook indiscretion but to confront it and resolve it in a way that brings the greatest good.
The only person who gets pissed about this is Norn, and some anons.

fertility mode sylphie might be the oldest looking young girl who's still beautiful yet