I went to my first musical and also my first opera at university. I was really really nervous and self-conscious as those environments were so alien to me, but was relieved to see that my friends (who were from far posher backgrounds and had attended these things all their lives) were dressed in jeans.
I became a frequent theatre/opera goer over years at uni, in no small part thanks to the student discount! (I also became a museum buff, once more always clad in jeans and a nice sweater.) If the dress code was still a tux or gown or something (which many people, usually older, did wear), I would have found it quite forbidding.
Behaviour is one thing – always behave respectfully – but I think getting dressed to the nines has outlived its purpose as a class signifier from last century. In fact, the cast members themselves (I know a few performers now) themselves are likely of a very liberal persuasion and don't believe in hoity toity dressing up lol