I was wondering if anyone knows this story:

Rarity and Spike enter a relationship and first tell Twilight, who disapproves and to ensure they don't do anything inappropriate she casts a spell on them which causes them to not be able to so inappropriate actions.

It was a One Shot and either rated Teen or Mature.

Please help me if you can, thanks in advanced.

The Loophole.

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This sounds like an interesting writing idea. I might try my hand with a different version.