Why do men and women view an adult watching cartoons as weird and childish?

I think it's because early cartoons were aimed at children, specifically. Cartoons nowadays have more adult themes and references than cartoons from the early 1900's to the 1960's. Those were cartoons from a more innocent era. People are more cynical and savvy now. There are plenty of cartoons nowadays that are aimed squarely at teens and adults. 'Family Guy', 'Futurama', 'The Simpsons', and many more, are written with an adult sensibility. I am not, in the least bit, ashamed to view these cartoons. In fact, I often reference them in everyday conversation. Despite all of this, I agree that there is a stigma attached to adults watching animated shows.