If you want to go down that road then I suggest you start looking at injuries for spare-time daredevils without insurance first (and professional athletes, but I think most of those have private insurance anyway).
The group that most commonly get mentioned are smokers. The reply smokers give is to mention that they are taxed so heavily for their habit that they are paying their way. It's also mentioned that because they die younger, they effectively subsidise other people's old age.
Yeah, that's a nice way to solve it actually, but I suspect that taxing sexual intercourse based on how frequent it happens and with whom it happens might just be a bit difficult. :P