these utility functions form tiny fraction of all possible functions, so if you don't think your own function is of this type then it probably isn't.

I generally agree with your conclusions in this comment, but I don't think that this is correct reasoning. The fact that a certain type of utility function is a really small fraction of all possible utility functions is not strong evidence for the conclusion that P(your utility function is not of this type | you don't think it is) because there may be a certain tendancy in human utility functions towards a certain type of function, even though functions of that type occupy an infinitesimally small fraction of function-space.