I don't write romance, so someone with more romance experience ought to weigh in (Elle - on hiatus?)

That said, my thought on this is that it depends on the story. The protagonist is the character experiencing the conflict, right? In a story of unrequited love, the character who loves the other would be the protagonist, while the character who does not return the other's love would be the antagonist (even if they end up together in the end... because the trial of earning the other's love is the whole story.) In a story where both characters want to be together but the universe (or other characters) conspire against them (e.g., Romeo and Juliet?), then the two lovers would be the protagonists and the universe / other characters would be the antagonist. Those are just two examples, but they demonstrate that the genre alone doesn't dictate who your protagonist(s) is(are).

Quick notes related to Prep:
1. Just because you only have one protagonist assignment doesn't mean you can't have more than one protagonist. You can use the "minor character" assignment for anyone, and of course use freestyle brainstorming to develop even more characters.

2. Just because you decided on the Protagonist Profile day that Juliet is your protagonist, that doesn't mean that you can't change your mind later and decide Romeo is (or both of them, or Capulet is - can you imagine your young teenage daughter getting caught up with your enemy's kid and then committing suicide over him??) That's why we Prep - to spend lots of time thinking, brainstorming, analyzing, organizing, and deciding what we really want our story to be.

Cheers,
Michelle