Molly asked a question about leggings and it's a good discussion:
In the '80s and about 11 years ago when leggings came back into fashion, they were like HOSIERY, and that's when I don't think you should wear them like pants. They were more like an accessory - like hosiery is - and I believe they should have been worn that way.
Over the years, leggings-jeggings-leggings (the items morph into each other), are a LOT more substantial. Sometimes thicker than pants. AND with the ever so popular Athleisure Trend in the US and Canada, wearing leggings as pants is a 100% acceptable look. It is not a faux pas at all. I'll go as far as saying that Athleisure is part of US culture. I see leggings worn as pants many, many, many times a day with ALL sorts of tops - especially on women up to the age of 30. It is THE bottom on women at the airport.
I wore leggings in the '80s and as recently as 9 years ago as an accessory under dresses/skirts when they were the hosiery kind. They were a way to keep warm and add interest to an outfit. And my '80s loving self loved it. See VERY OLD pics. I would do it again because I love hosiery.
Apart from the '80s look, I do not wear current and thick leggings as casual attire because Athleisure is not my thing.
ETA: After below's discussion, we have THREE ways you can wear leggings:
1. As Pants - butt and more is uncovered.
2. As Leggings - butt and some or all of the thighs are covered
3. As Hosiery - with another bottom like a skirt, long dress, shorts or pants.
How do you wear leggings?
I like to wear leggings as hosiery.