Show respect for the performers?
If I pay $250 for a ticket to see "Tosca" at the Met, I'll wear whatever I damn well please.
Respect for the performers? Fine, when Jose Carreras learns to show respect for tempo and the conductor's beat, and Barbara Daniels has respect for staying on pitch I'll worry about showing respect for them.
besides, I did a mucical in high school once. You can't even SEE the audience past the first few rows. The rest of the house is a black void.
Certain settings require certain clothing. Wearing jeans to the opera is like wearing a suit to a heavy metal concert.
People used to be raised with an understanding of what was appropriate in certain settings. That seems to have gone by the wayside in our culture.