Aha! I have a counter-point (I'm not trying to start an argument, honest. but when I thought of this I couldn't hold myself back.) Among humans your quite right, But you'll recall that Rarity and Applejack shared a bed together without wearing any bedclothes. I suppose you could say that Twilight didn't have any spare, but that still shows it okay among ponies.
Still though, one do have to wander why Celestia is reading Twilight’s diary and what she found there?
Well, that's kind of a different situation becaus a) they weren't wearing anything to begin with so there was nothing to remove, unlike Celly who wears a bunch of royalty-stuff. Of course, really she's only half-naked now because she's still got the necklace and big crown thingy. Oh, yeah, and b) they weren't sneaking into Twilight's bed, which makes it a different situation.
Well, obviously it's for Molestia-related reasons. That's why she's half-naked and in Twilight's bed.