let me remove the labels so we can make the ethical considerations more clear.
lets say a wants x and b wants y. when only x is available, a is happy and b is unhappy. when only y is available, a is unhappy and b is happy. lets further posit a world where both a and b can choose either x or y. in this world, it follows that both a and b are happy.
now lets add a twist. lets say that while a is happy with x and b is happy with y, b is happier with y than a is with x. in this world, when both a and b can choose either x or y, a is happy and b is even happier.
i hope it is self evident that in the first world, if either a or b prefer that only x or only y should be available to both a and b. that they are behaving unethically. perhaps it is less evident that in the second world it would be equally unethical for a to prefer that only x is available to both a and b. but it really shouldn't be.