[C]ritics have complained that contractarians fail to provide any good reason for thinking that acts prohibited by the norms agreed on by the hypothetical contractors must be wrong. It would be absurd, they argue, to suggest that this is because hypothetical contracts (that is, contracts that one has not in fact made but would have made under certain circumstances) are somehow morally binding. Contractarian constructivism is clearly not committed to such an absurd view. Although it makes the wrongness of lying a consequence of the fact that lying would be prohibited by the norms agreed on by the contractors, this is not because an obligation to refrain from lying is created by such an agreement in the way that promises create obligations. Rather, the wrongness of lying (like the wrongness of breaking a promise) is a consequence of this agreement simply in the sense that being prohibited by such an agreement is what its moral wrongness consists in.
Ronald Milo, ‘Contractarian Constructivism’, The Journal of Philosophy, vol. 92, no. 4 (April, 1995), p. 196