My girlfriend and I went to a piano concert at the Overture Center in April. We both wanted to be comfortable, so I wore a t-shirt and jeans and she wore leggings and a sweater. Little did we know that it would cause such a problem. Our seats were right next to a group of people that acted like we had the plague. At first we had no idea why they were staring at us, but they started to make comments about our appearance in a way that we knew they wanted us to hear them. Comments included "This is not a beer party" and "Why would you think it would be OK to wear that to the symphony". The rest of the evening included eye rolls, annoyed glances and audible whispers about our choice in clothes. We left the concert feeling like outcasts and embarrassed.
Is there a dress code at classical concerts? Were we in the wrong to "dress down"? It was really a surprise to me that our attire would be the talk of the evening.
I once went to a performance at Lyric Opera of Chicago's performance of Wagner's Götterdämmerung wearing work boots. It was winter in Chicago and my feet get cold, especially when parking was 10 blocks away.
I got looks. It obviously bothered people. In fact, I experienced the same whispering, eye-rolling disapproval it sounds like you received.
Oh, by the way, I was there to enjoy the opera. I was really looking to see this masterpiece that is rarely performed by such a high profile big time operation like Lyric. Because of this, it didn't bother me one bit that I ruined someone else's experience because of my footwear. Obviously, they weren't there for the experience. I can't tell you why they were there and my guess is they probably left at the 2nd intermission, but I can tell you the weren't there for the opera in and of itself.
My recommendation is to pay no attention to people who are obviously not there to enjoy music. Wear what you are comfortable in. IN the unlikely event an usher kicks you out simply because of your attire, make sure you send a note to the Artistic Director, Executive Director, Board President and any donors you can find. Copy them all on the same email! Let them know that you really enjoy classical music but were not given the opportunity because of your clothes. And that it is a real shame, because that is the type of entertainment you like spending your hard earned dollars on.
But that is an unlikely scenario. I know too many people who run these types of organizations that would have no problem at all with your attire. But unfortunately you can't control the reaction of the assholes you are seated next to. Ignore them while they stew in their own foul sensibilities while you enjoy the performance you paid to see.