This is kinda silly and should be obvious: I don't believe this myself as I'm not religious, but if someone is a conservative religious person and believes that homosexuality is "evil" and such people should be forcibly "converted" or whatever, voicing that opinion at work to actual LGBT people is not going to go over well. Of course, that opinion is anti-diversity and anti-inclusion, but threatening to fire people with these opinions is itself anti-diversity and anti-inclusion, because it's excluding conservative religious people.

To go full Godwin, firing people for expressing outright fascist viewpoints is also anti-diversity: it's excluding fascists.

Basically, there's no way to have true diversity and inclusion for all viewpoints, because so many viewpoints are mutually incompatible. There's simply no way to reconcile the viewpoints of two people, one of whom is LGBT (or at least pro-LGBT rights) and one of whom thinks such people should be marched into death camps. This really goes for any kind of viewpoints; you can only accept so much diversity of opinion before you get into an irreconcilable conflict.

In normal polite society, we avoid this problem by simply forbidding discussion of certain topics in certain places. But that means people cannot feel safe expressing themselves fully in those places; there's simply no way to allow that as long as diversity is allowed.