If I say "I prefer not to be tortured more than you prefer a popsicle", any sane human would agree. This is the commonsense way in which utility can be compared between humans. Of course, it isn't perfect, but we could easily imagine ways to make it better, say by running some regression algorithms on brain-scans of humans desiring popsicles and humans desiring not-to-be-tortured, and extrapolating to other human minds. (That would still be imperfect, but we can make it arbitrarily good.)
This isn't just necessary if you're a utilitarian, it's necessary if your moral system in any way involves tradeoffs between humans' preferences, i.e. it's necessary for pretty much every human who's ever lived.
that's more like being conditional that we cooperate. If my enemy would say that I could find it offensive and it doesn'ty compel me to change my actions. If you try to use utlitarian theory to (en)force a cooperation the argument doesn't go throught.
As per a recent comment this thread is meant to voice contrarian opinions, that is anything this community tends not to agree with. Thus I ask you to post your contrarian views and upvote anything you do not agree with based on personal beliefs. Spam and trolling still needs to be downvoted.