Given the explanation, I don't fault you one bit for putting your foot down on "Marshmallow". And I don't see any "macho posturing" about it. "Please don't curbstomp my verismilitude" sounds more like it.
Back on track:
In a 2nd ed game I ran, I had a bard (wannabe-archer) enter an archery contest at a tournament. In the preliminary rounds, the contestants got to fire 3 arrows at a target, total their score, then move up or be eliminated. This guy made it to the finals, but in the finals, the rules were different, and they each got 1 arrow at a time. Now, I must mention that in this particular game, the players only rolled dice for initiative and hp upon leveling, everything else was narrative and I rolled all the dice. He went first and scored well. The next roll was even better (natural 20, IIRC), but it wasn't his arrow, it was the next guy's. I had described the scene, and explained the change in rules, but he was not paying attention and got all freaked that the natural 20 was not his.
"That's my roll! I rolled that 20. That was MY roll!"
To this day, the other members of that group will drop that quote on me, even though we don't play "DM rolls all the dice" anymore.