Might I suggest tabooing "utility function"? The phrase is used in this community at various levels of abstraction, and useful responses to your question depend on which level of abstraction you intend.
No, a utility function is a perfectly well-defined mathematical object. The question is what it means for a human to "have" such an object; so "have" is the word we should be tabooing.
Does Abraham Lincoln have a six-dimensional vector space? Does Spain have an abelian group? The question "do humans have a bounded utility function" is kind of like that.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.