You can absorb the chakra of fire jutsu.... via inhalation... insides are also fire proof (more or less)... blaa blaa blaa
If you go by that logic most Katon users would be dead due to their insides being burned... like Sasuke, Itachi, Madara, 3rd Hokage, Old Pervy Sage....
'Fire Release (火遁, Katon; English TV "Fire Style") is one of the basic elemental nature transformations. It is performed by molding superheated chakra inside the stomach before releasing it via the lungs and mouth.' ~ Narutopedia
The chakra, perhaps, is released via the lungs and mouth, but the actual fire would have to be sparked after leaving the sensitive areas of the body. If the insides truly were fire proof, then Katon users would be nothing short of immune to fire. It is principle that Katon users are not immune to fire simply due to being able to use it well (resistant is one thing, taking a fireball to the face and having minimal burns is another) or having it as their prime element.
I am not saying that superheated chakra is less dangerous than, you know, some fire that you breathe in, but if we go by the logic that katon cannot harm the lungs and mouth of a Katon user, then being able to inhale fire would be a trait that almost any specialized Katon user can use to spit fire back at their opponents. I have yet to see someone get away with it in the zones, or the series for that matter.
Sage mode hardiness making the insides even more resistant, if you look at it that way, might would be enough in moderated doses, but I still don't think inhaling gunpowder that is then detonated by a spark, or a stream of fire for that matter is good for your lung health.