Theland - // Oh no not I'll mannered. Just didn't know how to react. A bit shocked. //
'Smirking' and 'Being Incredulous' as you put it, are examples of simple bad manners.
I remember dining on a cruise where we were seated with a couple, the man of whom advised me that anyone considering voting to leave the EU was obviously an idiot, and did not deserve a vote.
I did not smirk, and I was not incredulous, because I possess good manners.
It's not difficult to hide an adverse reaction if you possess the basic courtesy to do so.