When it comes to promise rings, I find them to be stupid.
What about promise rigs makes you think they are stupid?
I'm allergic to rings.
Just because promise rings are not for you, doesn't mean that they are stupid. There are some people that really value the meaning behind them.
I just don't like them and they make me nervous.
It's fine that you don't like them and you should definitely avoid all rings due to your allergy, but saying something is stupid just because of your personal preference is a bit rude. You might make people upset by insulting something that they actually truly enjoy and appreciate.