  At very large energies and in $SU(2)_L\otimes U(1)_Y$ gauge theories, the
trilinear gauge boson vertices relevant for $e^+e^- \to W^+W^-$ scattering are
related in a simple way to the gauge boson self-energies. We derive these
relations, both from the requirement of perturbative unitarity and from the
Ward Identities of the theory. Our discussion shows that, in general, it is
never possible to neglect vector boson self-energies when computing the form
factors which parametrize the $e^+ e^- \to W^+ W^-$ helicity amplitudes. The
exclusion of the self-energy contributions would lead to estimates of the
effects wrong by orders of magnitudes. We propose a simple way of including the
self-energy contributions in an appropriate definition of the form factors.
