While an arbitrary utility function can in principle occur, an intelligent entity with a self-contradictory utility function would achieve greater utility by modifying its utility function until it was less self-contradictory.
To the extent humans have utility functions (e.g. derived from their behavior), they are often contradictory, yet few humans try to change their utility functions (in any of several applicable senses of the word) to resolve such contradictions.
This is because human utility functions generally place negative value on changing your own utility function. This is what I think of when I think "reasonable utility function": they are evolutionarily stable.
Returning to your definition, just because humans have inconsistent utility functions, I don't think you can argue that they are not 'intelligent' (enough). Intelligence is only a tool; utility is supreme. AIs too have a high chance of undergoing evolution, via cloning and self-modification. In a universe where AIs were common, I would expect a stranger AI to have a self-preserving utility function, i.e., one resistant to changes.
Human utility functions change all the time. They are usually not easily changed through conscious effort, but drugs can change them quite readily, for example exposure to nicotine changes the human utility function to place a high value on consuming the right amount of nicotine. I think humans place a high utility on the illusion that their utility function is difficult to change and an even higher utility in rationalizing false logical-seeming motivations for how they feel. There are whole industries (tobacco, advertising, marketing, laws, religions, ...
This is our monthly thread for collecting arbitrarily contrived scenarios in which somebody gets tortured for 3^^^^^3 years, or an infinite number of people experience an infinite amount of sorrow, or a baby gets eaten by a shark, etc. and which might be handy to link to in one of our discussions. As everyone knows, this is the most rational and non-obnoxious way to think about incentives and disincentives.