  It is shown that in extensions of the standard model of quarks and leptons
where additive lepton number $L$ is broken by two units, so that $Z_2$ lepton
parity, i.e. $(-1)^L$ which is either even or odd, remains exactly conserved,
there is the possibility of stable dark matter without additional symmetry.
This applies to many existing simple models of Majorana neutrino mass with dark
matter, including some radiative models. Several well-known examples are
discussed. This new insight leads to the construction of a radiative Type II
seesaw model of neutrino mass with dark matter where the dominant decay of the
doubly charged Higgs boson $\xi^{++}$ is into $W^+W^+$ instead of the expected
$l_i^+ l_j^+$ lepton pairs for the well-known tree-level model.
