This is exactly what happens with conversations with my mother. Saying "yeh, thats 'cause he/she is fat/obese" is like saying c**t, theres an audible gasp. Then the stare of disbelieve,  an awkward "you cant say that..." and then I reply immediately asking why I can't say that, its the truth and the remark is ignored and subject is changed.  I feel like a three year old sometimes who says a naughty word. Or a student at Hogwarts muttering Voldemort.