@The Dark Pony  
Unless we have more observations about Rarity’s butt in particular, we need to take this elsewhere.

@BADASSassin  
Pinkie, definitely, wouldn’t be surprised for Rainbow Dash or Applejack either. Or Sunset even. Fluttershy though, you’d expect her to have some discomfort with it, yet both versions of her seem to love buttplay. But actually, there was a moment in Holidays Unwrapped where Rarity has Pinkie carry her somewhere, and Pinkie puts her hands on Rarity’s butt as she does so. (Here’s a picture of the scene, I think the only edit is the text)
 
As for Chrysalis, she takes pictures of the Main 6’s faces, except Rainbow Dash, she takes the picture of her from behind, and maybe it also counts for something that she takes a hair from her tail instead of her mane like she does for the other five, as if she wanted an excuse to get up close behind her again. Then there was the moment where she levitated Tirek in front of her to visually illustrate the part in her song where she says “how bad you got your rear end handed to you”. But on the other hand, she did seem uninterested in Twilight shaking her butt in her face. But given how she singles Rainbow Dash out, maybe she’s only interested in her and Tirek in that way.

@The Dark Pony  
I still feel like Fluttershy and Pinkie are more open to that kind of touching while Rarity would still think it uncouth.
 
I need to rewatch those episodes because I don’t remember that at all. Though I kinda headcanon that Chrysalis pays close attention to every detail of other’s anatomy (even private) in case she needs to replicate it later. She would find the image above invaluable in getting the pillowy softness of her butt just right to mimic Rarity’s.

@BADASSassin  
EqG Fluttershy did do a rump bump with Pinkie Pie.
 
Chrysalis seems to take an interest in both Rainbow Dash and Tirek’s butts in The Mean 6 and Frenemies, respectively.

@The Dark Pony  
All those instances only have to do with pony Rarity, whose butt is normally exposed. Anthro Rarity probably has greater qualms about having her butt touched. However, I still think she would greatly appreciate a butt massage (or just a back massage that happens to include butt touching) from someone she felt comfy with.
 
(Why is Chrysalis the exception there?)

I’d only give her one with permission. But she didn’t react when the Diamond Dog slapped her butt (she only took offense to him calling her a mule), and aside from hitting Twilight with her butt in Sonic Rainboom she’s put her hooves on Twilight’s butt to push her forward in the final episode, if I recall. I wouldn’t be surprised if she actually gets butt massages at the spa, though of course she doesn’t view it in a sexual sort of way, nor does anyone in Equestria except maybe Chrysalis.

@rautamiekka  

 
 
Clearly her standards have slipped. Diamond Dogs indeed!
 
Harumph I say! Harumph!

@polar_storm  

 
At least judging by recent pics (not sure they’re on Derpibooru) she’s been having quite a bit of fun with the diamond dogs lately :P

@rautamiekka  
Without permission for certain. Barbarian. Like one of those diamond dogs or something.

@The Dark Pony  

 
She might think that’s uncouth :P

T’is an ass if I do say

@BADASSassin  
I wouldn’t mind her facesitting me either

This is the kind of pic that makes you feel deep regret over having been born in a universe where Rarity’s gorgeous derriere doesn’t exist and will never be sat on your face.

Although I ain’t no Rares fan, a good ass is a good ass :P

I’d give her a butt massage, I’m sure she wouldn’t mind.

That’s one nice alabaster flank!

Wow! Can’t wait for the finished product!!

Rarity has the Best Butt <3

Now that is a nice Marshmallow butt~

Move over Celestia, Rarity is the top PAWG in MLP.
 
Would love to get two firm handfuls of that ass.

Butt of a goddess~

I would love to smooch those cheeks

Shines a like a pure diamond.

Nice booty  
boops it

“Look at my butt.”