OK, so what is wrong with "naturally refused"?
Nothing at all, although it carries a different kind of meaning to flatly or sharply.
On the other hand, sharply and flatly carry different kinds of meaning too.
Isn't is strange that refusal may be natural, flat or sharp.
I was offered a job as an unpaid, temporary, assistant drain-cleaner.
I naturally refused it.
... Of course I refused it, anyone sane would have done the same.
I flatly refused it.
... I very emphatically, but politely, refused it.
I sharply refused it.
... I refused it rather as if I had been insulted by the offer.
Have I got sharply refused right here? It's not something I would say normally.