I’m not convinced this is necessarily sexism (though I am not saying it is not), mainly because I hear this all the time and I am male.

I don’t do casual Friday and I wear what I call “business goth” most of the time. I wear dark, dressy pants and a dark, formal collared shirt nearly every day.

I also work in IT where often wearing shoes is considered “dressed up.”

Most weeks I get the “Why are you so dressed up?” Or the, “You know, you don’t have to wear that. You can wear jeans” statements about 2-3 times a week, and sometimes more often than that.

I’m pretty sure that most of my co-workers know I am male, so I don’t think sexism explains that.

I don’t wear jeans. I don’t even own jeans. I wear what I want to wear.

I think humans are just naturally good at singling out anyone different/deviant for attention and covert criticism, although of course this can be combined with sexism.