By and large the opposition benches have been wearing masks, haven't they? I know some Tory MPs have too, although very much the exception (and they'll never get a promotion!).
No reason at all why they shouldn't mandate mask wearing in Parliament as long as we have it in public spaces elsewhere. It'd be different in their office and socially distanced spaces of course, but eh benches are neither of those.
It seems to be seen a a marque of masculinity to not mask up and stare down the virus. Very odd, the fkn idiots.