Peter Singer, in an interview with Christine Smallwood at The Nation:
You're a utilitarian. Utilitarianism tries to maximize the net surplus of happiness over misery in the world. What if billionaire Larry Ellison's yacht makes him really, really happy?
This is what some call the utility monster argument. We would have to assume that Larry Ellison actually has capacities for happiness that are vastly greater than anyone else's. Ellison's yacht cost $200 million, and if we assume that $400 can repair an obstetric fistula, that means that the suffering relieved by 500,000 obstetric fistula repairs is not greater than the happiness that Ellison gets from his yacht. That, I think, is not physically possible. But if we ever encountered Martians who could convince us that they had a vastly greater capacity for happiness than we do, then it could be a problem.
Then the moral position would be to let the Martians colonize Earth and make us their slaves.
Yes, that does seem to be the implication of the theory. A lot of people do think that's a damning objection to utilitarianism.