There's no end to that road. What people do in the bedroom affects their health as well. Shall we tax the more risky sex practices?
Although hard to imagine, if we can somehow develop a system to charge people more for unsafe sex while perfectly anonymizing it, I do not see how it would be different from carbon tax in principle.
This could obviously be deemed a bit of a slippery slope fallacy, but I'd argue the above is probably more beneficial than taxing my soda if you truly want to reduce healthcare costs by having healthier individuals. But, who wants to go that far?
As for the bedroom thing, this is a bad kind of slippery slope argument (not all slippery slope arguments are fallacies). It's ignoring the fact that penalizing soda is not a binary +health/-health decision but a complex tradeoff that includes the magnitude of expected gains and the probable sentiment of the public.