It is shown how the effects of molecular reorientation may be incorporated in a fully quantized quantum-electrodynamical treatment of a high-order nonlinear optical effect. Specifically, a general temporal theory is developed to account for the second-harmonic intensity produced through phase-conjugate six-wave mixing. The theory permits elucidation of the intensity of the second-harmonic radiation for arbitrary arrangements of the generating laser beams and molecular geometry. Several models are considered: a one-dimensional model, linear geometry, and a planar geometry. A comparison is made between the results associated with these models and with those obtained from ultrafast experiments on dilute solutions of substituted stilbenes. We find that the off-axial components of the molecular polarizabilities are necessary to properly describe the orientational dynamics of such molecules.