Or for Swan Princess au, the two heirs are destined to be wed when old enough (like stand by you) but they hate being forced to spend time together. Continuing til Bog is in his early 20s and Mari is just past 18, they finally fall for each other but Dagda decides "you know what I'd rather have her marry the Southern Kingdom's son and not this twig boy". The curse is basically only true love can save her from being a moth (moth monster?) forever?
I can actually see it, when their age span is only two/three years that they are more annoyed by each other. I can imagine that in their late teens they start to be more friendly to each other and start to practice fighting together. And when they are at the each you said, they actually for each other.
I can imagine that Bog offends Marianne just like Derek did to Odette, with the thing what more should be there than beauty? But he mostly says it, because he thinks that’s what the most important for Fairies.
Dagda is also offended by Bog’s disrespectful behavior towards Marianne and also uses this as excuse to call of the arrangement. Back in their Kingdom a young man named Roland presents himself as Prince from a Kingdom far away. In truth it’s the sorcerer who Dagda has banished years ago. Marianne doesn’t fall for his charms and also doesn’t budge under the please of her father, so the sorcerer kills the King and takes Marianne away.
Moth sounds something good to be cursed to. Or something else that could be seen as hideous by Fairies, but beautiful by Goblins. *shrug*
Of course Bog tries to save her, as soon as he hears about this and his able to lift the curse from her by saying why he actually loves her.