We were certainly never allowed to use the word belly when I was a child.
I clearly remember a class-mate cringing when she had to read aloud from Genisis about the serpent going on his belly.
Tummy and bottom were acceptable, but not bum. Women and girls certainly wore panties in the Glasgow area, where most people considered knickers rude.
Most adults used tummy even when talking of their own abdomens.
As far as I am concerned everyone should be able to use or not use any of these words as they please, just as long as they don't want me to talk about my belly, or anyone else's.
Why would gusset liners be unacceptable to you,skunkhair63? Admittedly, lots of garments have gussets, but surely we would get the point regarding where to place them?