Firefighting can at times qualify as a natural monopoly, depends on things like population density. It has absolutely nothing to do with the idea that firefighting should be provided as a free government service.
Health care as a whole, however, is definitely NOT a natural monopoly. It's possible that, in certain markets, certain services might be, depends on the marginal economics of the business (driven by factors such as density for ambulance services or services requiring extremely costly imagine equipment).
It's extremely unlikely, on the other hand, that the market for primary care physicians is a natural monopoly (since the entry costs for a new provider are very low, removing one of the most important prerequisite for a natural monopoly).
Might want to read this piece (although I'm afraid it's full of analysis and rather short on ranting):
I'm a huge fan of the ACA, and I'm glad SCOTUS upheld it. If you're going to support it, though, you should actually have some understanding of how the system it's attempting to reform works, particularly since, even in countries with single payer (like the UK's NHS), there are many competing providers.