Simply put, it implies a lack of regard towards the subject as human. Saying «Female» over «Woman», «Lady» or «Girl» invokes images of animals rather than humans.

Same goes for men, but its a lot more uncommon and sometimes actually a positive, as «Male» invokes virility. Its honestly a weird double standard.

“female protagonist” good

“I talked to a female today” bad

Short version but depends if you’re talking about a specific person(s) or describing the person.

In my opinion, it’s more of a lab type word when talking about biology or anatomy.

But it’s not actually all that offensive. Women have so many negative terms used to describe us already. Female can stay neutral. And I will continue to use it.

People give way too much power to small vocal minorities. Don’t let incel neckbeards make another negative term to describe our sex.

It has the potential to be dehumanizing if used as a noun, because it’s usually a scientific word reserved for non-human animals.

Moreover, it’s use is often considered sexist because people will write messages such as “men and females” showcasing a double standard in which male people are deserving of enough respect and humanity to stray away from the scientific “males” whereas female people are not.

Generally, accepted usage of “female” includes the following

• as an adjective (ex: my female supervisor)
• if you also use “male” as a noun
• non-human animals
• military settings & forms

It can be dehumanizing since “female” could refer to any sexual creature, not necessarily a human being.

So in the context of biology, female is appropriate. We may be talking about the properties of ovaries, the effects of high estrogen, the process of pregnancy and childbirth. We may be talking about human females, but potentially also just females in general.

But in the context of people, female is inappropriate, and it is better to use terms that affirm the subject is human: woman; girl; lady. A woman is (usually) a female, but a female isn’t always a woman, isn’t always a human, and so by referring to women as “female” you subtly suggest that their humanity is absent or unimportant.

the problem is with referring to women as females

Nothing or a lot depending on use.

If it’s being used where “male” could/would be used all is well.

If not it’s often dehumanizing.