Hopefully there won't be an 'embarrassing' situation...

I think you know what i mean by 'embarrassing'

T-That i-is... H-how can you be so cruel?

It is quick and completely painless. What's cruel about it?

He was a mountain of muscles that made even Bulk Biceps look puny in comparison.

Hm doubtful...
While the Space Marines ARE big and muscled, they are NOT muscle bound.
Bulk Biceps is so overmuscled that he CAN'T move well.
Obviously the single muscles on Antonius are bigger than the one on Bulk Biceps merely because he is THREE TIMES the size of a Pony...[even though that seems a bit small for a Space Marine, or this ponies are quite big.]

8032194
Well, it's quick and painless if it's a headshot. Anywhere else? Not so much.

9050097
Considering the damage a bolt does? I'd say for anything smaller than an Astartes it'd be painless.

"T-That i-is... H-how can you be so cruel?" panted Fluttershy.

I'm surprised. This is Fluttershy. She deals with animals all the time. Including carnivores with sharp teeth, who would eat you alive. Compared to that, a quick death from your head exploding doesn't sound so bad.

"Oh... manticore?"

Hahaha I manticore your head off lol

8391022
Likely referring to the fact that Astartes don't really have any fat on them, so there isn't anything smoothing out the lines. That, and Astartes would be taller than Bulk, who is rather large for a pony, so to Rarity, Antonius would be positively massive.

"What the hell?" he asked, "what are you doing in the tub? How did you even get into it? There was neither a sound, nor could I feel a wave of displaced water!"

Pinkie is an Agent of Chaos, she can so that...
Wait... not THAT Chaos, put the bolter down!

Antonius used one of the provided towels to clean his armor and thought about his hosts. He still had no idea how the pink menace had managed to get into the tub without him noticing. Then, he got 'visited' by Rarity. Well, it certainly didn't get boring around those ponies.

PINK MENACE BWAHAHAHA genius! XD

How the ponies could stand that amount of sweets was incomprehensible. Were they immune to sugar?
Their powered by it, well at least the pink one is.