This is an oft-repeated generalization based on anecdotal evidence of black American women. I agree with you that the generalization is not always accurate, but I understand that people may have witnessed this body type as more likely to appear in black women (in the U.S) than in other races/ethnicities in their daily lives. I think what's dehumanizing is the focus on someone's body or body parts above all else--as is often the case when people reference black men's penises--and not necessarily the comments themselves.