The Onion

Using "male" and "female" as nouns is offensive because it dehumanizes the subject and reduces them to their gender.

A "woman" is inherently human -- specifically an adult female human. By calling that person only "a female" you remove their humanity and maturity, leaving only their sex. This is why it's so common for creepy types, and why it's so creepy, because it betrays that only one of those three descriptors is important to them.