I think this is one of those bits of manufactured controversy that's not really about what it appears to be about.
First of all, gay and lesbian actors still face a lot of discrimination. You can point to Jodie Foster, Zachary Quinto, Luke Evans, or Neil Patrick Harris all you want but there are still loads of gay actors who get turned down for parts simply because they are gay. You'd be surprised how homophobic elements of the casting and TV production industry still is.
By starting a debate from a position where the majority of people will go 'no that's ridiculous, a good actor's a good actor' then it helps to shed light on the fact that the opposite position is similarly ludicrous; hopefully, levelling the playing field for gay and lesbian actors.
After all, if we don't think it's right that only gay actors should play gay parts then by extension it's not right that only straight actors should play straight parts.
The other thing is about shedding light on an empathy gap. Davies has a new drama series coming out about HIV and AIDS in the 80s. At the time, the growing health crisis was ignored for years because people were uncomfortable with discussing sex and sexuality in public. Indeed, gay men particularly were othered and blamed for the spread of HIV, while straight people were smugly told (by The Sun) that they couldn't get it through 'straight sex'.
So what better way to get people in 2021 thinking about how gay people were marginalised, ridiculed and ignored over HIV than to start a somewhat spurious argument about actors? Plenty of people posting a response to this, whilst correct in certain specific terms about acting qualifications, are showing themselves to be entirely ignorant of how gay and lesbian people's lives and experiences differ from straight people's. And this is how the HIV crisis grew to become what it became.