It is kind of complicated because the term female is not derogatory in and of itself, but in this context it is. We normally use males and females if we refer to animals, but men and women if we refer to humans. Because as humans we historically see ourselves as higher beings, more intelligent and such, and therefore referring to a human as one does to an animal is seen as offensive. In incel culture women are called females as a way to objectify them, as if they are animals not deserving of a different term. As if women as more simple creatures than men and don’t deserve to be given a similar elevated status over animals.

It’s hard to explain cultural context like this but i hope i helped a little.

The title would not have been offensive if it was “a female behind” instead of “a female’s behind” because then it would have been used as a way to describe the behind, not to reduce an entire person to only their gender/genitals.