Predictions for next time: Rarity sneaks around in preparation for what she believes to be a secret rondevous, backfiring predictably when she runs across Twilight who asks why she's been acting strangely. Continuing the current trend of hilarious miscommunication, Rarity confesses in a way that makes Twilight think she's started an excersize plan. Wanting only to encourage her friend, she agrees to join them, making it a, erm, 'foursome'.
You know, once you allow your mind to start thinking about these things out of context it's really difficult to switch it off.
All I can think now is 'unicorns are the horny ones'.