Can Bosons couple to gravity? Why do we need vielbein?

It is said that In theories such as Supergravity where there are fermions coupled to gravity, one must use an auxiliary quantity, the frame field (vielbein).

1. In supergravity, can a boson be coupled to gravity? If not, then why?

2. Why is it that because fermions couple to gravity, then vielbein must be used?

(2) We want to use vierbien to couple fermions to gravity because the Dirac equation is formulated in Minkowski space and we want the behavior of fermions in a general spacetime to be locally like their behavior in Minkowski space. Therefore we use the local frame fields/tetrads/vielbien $e_I^{\mu}(x)$ to write the Dirac equation as
$$(i \gamma^I e_I^{\mu} D_{\mu} - m)\Psi = 0$$
Of course if we use the constant coordinate $e_I^{\mu}$ this reduces to the usual Dirac equation.