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.

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.

"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.

the problem is with referring to women as females

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.