I agree that formal nights are formal and if you don't want to dress, then eat on the Lido deck. However, on non-formal nights, it does not specifically state no jeans. I grew up in KY/SC/TN and when you went to a Casual event - Jeans and a nice shirt were typical. I don't wear jeans anymore (too hot for them down here), so this isn't an issue for me, but if you are reading the dress code it gives things that are not allowed (shorts and tank tops), but doesn't mention jeans, then one can reasonably conclude that jeans are allowable. I am only talking about Carnival - I don't know other cruiselines policies.