The way Castiel inspected the marshmallow before eating it reminded the Doctor of himself, and thus saw nothing wrong or odd. The Doctor had been in his own share of situations on foreign planets (including Earth) where he'd been fascinated or questioning new customs or experiences, so even the question was taken as perfectly ordinary.
"Yes. Usually. And sharing food around a bonfire like this is exceptionally poignant, because it hearkens back to Earth's human primitive instincts, when instead of marshmallows and fizzy soda, they dined on dino-steaks. And let me tell you, those dino-steaks aren't what they're cracked up to be. Pretty bland, without seasoning. I had to teach them about salt."