  We prove that the values of the electroweak mixing angle $\sin^2\theta_W$ and
intermediate mass scale $M_I$ have vanishing contributions due to one-loop,
two-loop and gravitational corrections in Grand Unified Theories which
accommodate an intermediate trinification symmetry ($G_{333D}$) invoked with
spontaneous D-parity mechanism operative at mass scale greater than $M_I$. The
proof of theorem is robust and we verify the results numerically using
supersymmetric as well as non-supersymmetric version of $E_6$-GUT.
