Another one I'd add to your list: prudish American attitudes toward nudity.

I'm American, and I'm embarrassed about how so many of my fellow countrymen automatically equate nudity with sex -- and reduce the number of options available for those of us who enjoy casual communal nudity (even if it's only in same-sex spaces).

When you invoke "a Millennial thing," I don't know if you're referring to Gen Y specifically -- or just "millennial" as a lazy shortcut to refer to bratty little children in general.