Fallacy:
"We should act so that utility is maximized."
Response:
There are many problems with utilitarianism. First, measuring and comparing utilities is not actually possible—it's all hypothetical. This makes it impossible to impose in practice; but as with most state justifications, it is used as excuse based on the ruler's decision of what people's utilities are. It is an amoral (hence frequently condoning immoral infringement of rights) philosophy than can justify even so far as robbing and murdering some so that utility across a population is increased; it does not respect NaturalLaw.
Start with Rothbard's arguments in chapter 26 of Books/TheEthicsOfLiberty.
Also, a funny related comic.