How does morality get it's ability to be rationally binding? If the very definition of "rationality" includes being moral, is that mere wordplay? Why should we accept this definition of rationality and not a different one?

Also see Peter Singer's The Triviality of the Debate Over 'Is-Ought' and the Definition of 'Moral'. In short, a justification of moral prescriptions comes back to an explanation of why you should care about those moral prescriptions. Or on LW: The Moral Void.

A killer quote from Singer's paper:

Disputes over the definition of morality and over the "is-ought" problem are disputes over words which raise no really significant issues. [Of course,] lack of clarity about the meaning of words is an important source of error, both in philosophy and in practical argument… My complaint is that what should be regarded as something to be got out of the way in the introduction to a work of moral philosophy has become the subject matter of almost the whole of moral philosophy...

Now that I think about it, I'm going to make that the new epigraph for Pluralistic Moral Reductionism.