I'm 47 and have never been to an opera. Having said that I once sat through Carmen on Sky Arts 2 on a dull Sunday afternoon and absolutely loved it. The inverted snob piece is probably holding me back here, because I swore as a youth, that no matter what else I did in my life, I was never ever going to wear a dinner jacket. There is probably a way around this, so maybe one day.
As others have said being posh is a perception and you shouldn't necessarily be offended that others consider you posh. Take craft beer, you are happy to pay a premium, for what you perceive to be a premium product and are happy to let others know that they are satisfied by drinking inferior alcohol. Maybe at this point I am straying closer to that other label "The Snob". Would you consider yourself a beer snob and more importantly would you take offence at others perceiving you as a beer snob ?
Personally I think you can do pretty much whatever you want without it affecting your class. As I have mentioned the theory I follow suggests class is more dictated by beliefs and values and you can hold onto these whilst engaging in lots of seemingly conflicting actions. It also breaks down barriers. So, go to the opera as you seem to be missing out on something you'd love.
The snob thing is a different argument so I will save that for another day.