OK. Here's the deal. For some reason, your parents didn't know any better and never taught you it is disgusting to blow your nose while others are dining.
Did they teach you not to loudly belch at the table? Did they teach you not to pass gas loudly at a restaurant and then proclaim "Smell that"?

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Surely they taught you the above.
Well, blowing your snot while others are eating is just as bad. You just were never taught that. You just don't know any better, because your parents didn't know any better.
Consider yourself taught. Do not blow your nose where others are eating.