Being somepony's friend can require accepting the... quirkier parts of their personality: something Rarity is all too aware of, especially when it comes to the work involved in accepting hers. So discovering that Fluttershy possesses -- shall we say, a 'special interest' -- simply means that the designer needs to accompany her to the convention. Just to make sure the pegasus isn't going to get hurt.
But a mare of dignity and taste does not casually wander into a 'special interest' gathering and emerge unaffected. And as it happens, neither does Rarity.
Rated C for Conifer Crackfic.
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