Bottomline is everyone has different taste, and different upbringings. Lots of us grew up reading comics so to see it come to life is amazing! And that's just one of the millions of reason why 'grown ups' may like it. Maybe John grew up in a box and that's why he thinks that?
So tell John to stfu and ask him why he gets to decide what 'grown men' are suppose to be like? Sounds like John is the real person that isn't a 'grown man' for thinking that way.