I’m pretty sure this is what’s happening.

Rarity and Applejack are trapped in a room that they can’t leave unless they have sex.
They both dislike the idea of going that far with the other.
Rarity says “With people I like, this isn’t the first thing I’d want to do.”, saying the first part very quietly.
Applejack asks her what she had just said, as she believes that Rarity said something important in a quiet voice.
Rarity says not to make her repeat it, and the two start arguing. In their argument, Applejack accidentally confesses that she also likes Rarity.

Appropriate  

Really bad edit, the translation is very poor.