They don't actually believe that up is down, no.

What they think is anyone should be able to do anything they want, even to their own destruction, if what they're doing isn't hurting anyone else. So they play along and lie to themselves and others for the sake of this perceived "greater good."

This is why they're baffled when someone cares about another persons wellbeing in this regard. "Why do you care so much?" They'll say, to the person that actually cares about the wellbeing of others enough to say no to them.

The trick is, most of them haven't caught on yet to the fact that this troonery that's been let loose in the world is harming innocent bystanders to this cult of biodenialism.