  Generic SO(10) GUT models suffer from the problem that Planck scale induced
non-renormalizable proton decay operators require extreme suppression of their
couplings to be compatible with present experimental upper limits. One way to
resolve this problem is to supplement SO(10) by simple gauged discrete
symmetries which can also simultaneously suppress the renormalizable R-parity
violating ones when they occur and make the theory "more natural". Here we
discuss the phenomenological viability of such models. We first show that for
both classes of models, e.g the ones that use ${\bf 16}_H$ or ${\bf 126}_H$ to
break B-L symmetry, the minimal Higgs content which is sufficient for proton
decay suppression is inadequate for explaining fermion masses despite the
presence of all apparently needed couplings. We then present an extended ${\bf
16}_H$ model, with three {\bf 10} and three {\bf 45}-Higgs, where is free of
this problem. We propose this as a realistic and "natural" model for fermion
unification and discuss the phenomenology of this model e.g. its predictions
for neutrino mixings and lepton flavor violation.
