I agree, however we may consider ourselves adult and I certainly like to think we are, other people of advanced age are certainly not mentally. Social media tells us this every day, there seems to be more immature behaviour from the 'adults' than the kids sometimes!!
The point being we come from a demographic where sci fi is considered escapist, fun and quite often thought provoking, opening up questions about us and the universe around us. Not everyone is like this (hard to believe I know right?). They only consider what they know, the things that they see on reality shows and social media and see anything that edges towards fantasy and fiction as nonesense and childish. I have friends with wives like this.
Episodes of Black mirror like Callisto just reinforce these thoughts and actually create a divide between their percieved 'normal' behaviour and our 'childish, nerdy and abnormal' tendencies.
Its all nonsense of course but there none the less.
I personally had no issue with the episode par se. I found it a good story, amusing and the characters interesting, but did wonder what the 'normal' folk would think of it.
It would be interesting to hear the produces/writers point of view.