I feel like a lot of answers here are dancing around why people find it offensive without really addressing it.

As an adjective "female" is completely fine to distinguish between genders when applied to humans. As in "a female athlete" or when a form asks you to select "male" or "female" (ideally with additional options "diverse" and "prefer not to answer").

Where it's problematic is when it's used as a noun. In English "a male" and "a female" is almost exclusively reserved for animals. For humans we have "a man" and "a woman". Calling a person "a female" is often considered offensive because it carries the implication of women being either animals, property or at least so extremely different from the speaker that they don't consider them equal. This impression is reinforced by the fact that the trend of calling women "females" is popular with self-proclaimed "nice guys" who blame women for not wanting to date them when in reality it's their own behavior (for example calling women "females") that drives potential partners away.

So in itself, the word "female" is just as valid as "male" and in some contexts definitely the right word to use but the way it has been used gives it a certain negative connotation.

Female as an adjective is perfectly fine.

A female patient, a female politician, a female customer, etc. That's the best way to refer to those.

What's bad is using 'female' as a noun: "A female. "

In general, you just don't use adjectives-as-nouns to refer to people. You don't call someone "a gay", "a black", or "a Chinese". That is offensive, and "a female" has the same kind of feel.

(there are exceptions to the above: you can call someone 'an American' or 'A German", but not "A French". I don't understand why - if you can't feel your way, best just avoid it)

Now, you could get around it by calling someone "a female person" - except that we already have a word for "female person", and that's "woman". And to go out of your way to avoid saying "woman" makes you sound like some kind of incel weirdo, and you don't want that.

Not really offensive on its own, but it carries a reductive and dehumanizing vibe, depending on how it's used. And the ones who use "female" instead of "woman" are often incels and/or misogynists, giving the term a bad conotation.

Also, Ferengis...

It’s potentially offensive when people say men and females, which is often why it comes up online. Using either male or female as a noun is dehumanizing, in that it’s not commonly used to refer to people, but mostly animals (law enforcement and military use them as nouns, but they’re also intentionally distancing themselves from the people in reports).

Basically, “women” feels weird for a lot of English speakers, but “girls” sounds creepy, so they try for something else. Just go with women, 99% of the time, it’s perfectly fine

Context is important. If I say: "Sexual dimorphism is when a species has two distinct sexes, male and female," I dont think many would find that rude. Now, if I say "Im so sick of females telling me what to do" you might get some cocked eyebrows.

The way I explained it to a chronically single friend who used this word problematically all the time, and made him stop: Female is a word that describes gender and/or sex. My wife is female, and so is my dog. My wife is literally a woman, and my dog is literally a removed, so if I speak of my wife with the same sterile language that I speak of my dog, then my wife would easily conclude that I have no respect for her. I then asked him how the dating world was treating him, he said "bad", and I said "of course, because you treat women like dogs".

Never heard him say it again.

It really depends on the context. When used as an adjective, it's fine. For example, the sentence "My female coworker has brown hair." is correct. However, when it is used as a noun, it can be dehumanising. For example: "A female at my workplace has brown hair" is dehumanising. It can be used as a noun when talking about non-humans ("After mating, the female will lay her eggs.") or in medicinal context when referring to people with uteruses.

It's super context dependent. Asking "How do I ask a woman out?" Vs "How do I ask a female out?" say very different things about you.

Using it as an adjective in some cases is fine, never use it as a noun, unfortunately due to assholes using it that way it now has a negative conotation.

It’s kind of like the difference between talking about people who are black and referring to someone as “one of the blacks”. It’s subtle, but the latter is objectifying where as the former is descriptive.

It sounds fairly scientific in a crunchy or incel way. As others have highlighted here, there are times it can be used, but generally, I'd stick to "women" or "ladies" for most situations. It flows better and avoids potentially negative connotations. That said, if english isn't your native language, I'd expect native speakers to cut you some slack; english can be a difficult language to learn, and the language is always evolving, particularly around gendered language right now. Sounds like you are putting the effort into learning it, though :)

It’s an adjective not a noun when talking about people. The sort of people who use it as a noun tend to be misogynists and so when people do it they’re often unknowingly writing with a misogynist accent if that makes sense.

Mostly just by association. It sounds very incel-y.

"I have a female friend." (As in "I have a friend that's a woman.") "I've talked with a female today." (As in "I've talked with a woman today.")

The first one is fine, because isn't using the word as an adjective. The second one is derogatory, because it is being used as a substantive.

When used as a noun they're how you refer to non-human animals so when you use them for people it sounds that you don't think men/women are human

Male and female are biological terms. Mostly "man/woman" are more appropriate unless you're specifically talking about biological sex. Particularly since a certain bunch of people is now using "females" with a bit of underlying vitriol, it'd be a good idea to stay away from it.

male/female usually used for animals, for humans usage of man/woman would be the right way

Depends entirely on how you use it. There are many perfectly valid uses for the word, like in science or on a checkbox where you select your gender. The point where it can get offensive is when referring to individuals or groups of women as "females" where you could instead just use "women". It's language often used by incels and the like.

The problem is using it when "woman" is the correct word. If you say stuff like "the female at the customer service desk doesn't know how to do her job" then you run the risk of being called an incel or ferengi, though if you don't seem like a sexist, churchy, or maga-hatter, then you can probably get away with it by not sounding like a native speaker. Or just avoid all that like me and don't talk to people because social interactions of any kind are mentally draining.

Some people think it's dehumanizing. As an adjective, it's more acceptable ("There is a female nurse"); it sounds a bit off-putting as a noun ("The nurse is a female"). There are some people who don't like to use it at all, and that leads to awkward things like using "woman" as an adjective ("There is a woman nurse")!

You're probably okay as long as you stick to using it as an adjective, but you still might offend some people.

it's supposed to be an adjective (female dog, female human, etc.) but people who want to reduce people to nothing but their sex will use it as a noun.