There are two conflating issues in your example: privacy and economic externalities. Taxing soda does not violate anyone's privacy while monitoring sexual pract

There are two conflating issues in your example: privacy and economic externalities. Taxing soda does not violate anyone's privacy while monitoring sexual practices does. 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.