It doesn’t say “a female behind.” That would be fine. It says “a female’s behind,” using the noun form of female to refer to a woman. Normal people don’t refer to a woman as “a female” outside of technical contexts like medicine or science. In casual speech it comes across as dehumanizing to call a person “a female,” and this is a speech pattern that is primarily used by misogynists, especially the incel variety.

The preferred phrasing would have been “a woman’s behind.”

That said, giving the person a permaban over this seems pretty excessive unless there’s additional context.