Just to tl;dr arguments here for new readers:
- Most comments side with the idea that FL was not in the wrong for her actions because, honestly, adultery is never okay -- even if it's based on "love."
- In addition, the romance is not set in some modern era where the breakup has zero consequences -- the FL is a Duke's daughter (i.e. right under the Royal Family in terms of noble hierarchy), the Prince is a Royal, and the Antagonist Female Lead (AFL) is far lower on the noble hierarchy. Thus, a breakup of the initial match would doubtless have significant political ramifications that most stories enjoy ignoring. In such societies, duty to the nation trumps love, so the prince's love is moot.
- There is also the extensive training the the FL undertook to even be considered a potential Queen, all of which was neatly sidestepped by the AFL on the grounds that "the prince wuvs her." This essentially means that, had the FL not acted out, the kingdom would've likely had an untrained Queen at the side of an idiot prince who can't keep his mind out of his gutter every time he looks at his Queen. Not exactly the best recipe for a stable administration.
- Those who are in favor of the AFL argue that the FL should have let them be happy because either "well, it's love, right?" or they find that the FL went too far with having the ML life-swap with the Prince and dumping them into (presumably) miserable poverty to live out their adulterous lives together.