  Sterile neutrino dark matter of mass O(1-10) keV decays into an active
neutrino and an X-ray photon, and the non-observation of the corresponding
X-ray line requires the sterile neutrino to be more long-lived than estimated
based on the seesaw formula : the longevity problem. We show that, if one or
more of the B-L Higgs fields are charged under a flavor symmetry (or discrete R
symmetry), the split mass spectrum for the right-handed neutrinos as well as
the required longevity is naturally realized. We provide several examples in
which the predicted the X-ray flux is just below the current bound.
