The real cost of fiscal externalities running through the health system isn't the transfer from relatively healthy taxpayers to those choosing unhealthy lifestyles (and the otherwise sick), it's the cost we bear when we put too much priority on reducing that total health bill relative to other goals.
Recall that a fiscal externality is an effect I have on you by virtue of the tax system. If my behaviours affect my health, and even if I would have made exactly the same choices if I bore all of the costs of my own healthcare, I nevertheless impose a cost on you through the tax system. But as just about every choice you make has some effect on your health, the scope for individual autonomy is awfully limited in a world where we tried setting policy to minimise those behaviour-related costs. @LibertarianView provides a short list of things you might be doing that cost the UK's NHS money.
I fielded an email from the producer over at Freakonomics Radio who wondered if I knew of any studies of the aggregate cost of sex; Stephen Dubner had a pretty tongue-in-cheek proposal around it a while back. I didn't know of any. I hope that nobody took Dubner's proposal too seriously as I'm rather sure he didn't. But I suppose a study tallying the costs of sex, under an assumption that there'd be no loss of consumer surplus were it abolished, could be just the thing to make the whole "Costs of X" industry finally die of shame.