It's an effective strategy to point out "don't say this about this selection of people if you aren't willing to start saying similarly about its opposite."

I.e. one might not realize a blanket statement about the habits of 22 year old men is incorrect/untenable until someone points asks one to do the same for women, at which case for whatever reason cultural training or mechanisms for avoiding prejudice kick in.

I don't see someone trying to be offended - I see someone challenging a baseless assumption about an arbitrarily defined group of humans. No matter how you draw those lines in your selection criteria, you're going to get it wrong for a not-insignificant group of that selection, so why bother? Just don't make such blanket statements.

Because group dynamics are not about getting it right for any member of a group -- in fact you can get predictions wrong for all members of the group and still be right in their overall behavior.

You can "get it wrong for a not-insignificant group of that selection" and still be right for the overall group behavior.

Likewise, you can get lifestyle/beliefs/etc wrong for "a not-insignificant group" of people in the bible belt, but you can still make accurate statements with predictive power for the area as a whole...