It is not an illegitimate art form lacking structure and requiring little or no training!
It is not exclusively for those who wish to perform! Only a small percentage of belly dancers actually perform in public.
It is not always done by women with great bodies in scant costumes! Many belly dancers (of all sizes) often choose traditional or traditionally inspired costumes to dance in. These costumes can provide full coverage from neck to ankles.
It is not hard! While it is true that it takes years to master all of the isolations, undulations and combinations of belly dance, the basics are natural movements which are, with a little practice and patience, relatively easy to learn.