Non-native as well but for me, "female" is an adjective and only used as a noun in technical setting. If used in regular conversation setting, it's condescending and dehumanizing.

Good:

• There are fifteen females in the survey.
• The female is more aggressive, typical of hyenas.
• You bought the wrong USB cable, this one is a female.

Bad:

• This female is not very good at her job.
• You females need to tone it down.

Female is still an acceptable term in some context: eg, when referring to the social group on a societal level, female can be fine, also for identifying someone's genetic/biological sex as "female" for medical/official contexts, that's still okay in most cases.

Where it's not okay is to use it on an individual level or to refer to a small group of ladies. The term is seem as cold, clinical, and in some cases, dehumanizing. It comes off as boiling down a person to their function in reproduction and nothing more. "You are the female and you carry children." Kind of thing. Like women are some kind of bakery for your crotch goblins, and not people worthy of respect.

But something like "the female population of the country" is fairly okay, since you're referring to the entirely of the people who identify as female, not an individual or small group of individuals.

At least, that's my take. I'm just some guy. If any women want to correct me, I defer to your judgement and opinion, and happily retract any contradictory statements I may have made. I am always happy to be corrected.

Short answer: It's a turn of phrase the Incel movement popularized as a dehumanizing dogwhistle.

Long answer: In current American English, "Female" and "Females" are highly formal terms and really are only commonly used in situations like law and academic discourse (this is true of "male" and "males" as well, though there's much less cultural baggage associated with those terms). People who use them in casual conversation instead of the much more common "Women" (or the diminutive, "Girls") tend to be the kind of person that uses formal language to emphasize their own intellectual superiority over the common masses, and in particular all the women who won't have sex with them. In fact and almost invariably, this is presented in the form of explanations about why nobody wants to have sex with them, and the cultural forces that are causing it, and why deep down it's the women's fault they can't get a date, and it just all goes downhill from there.

edit: clarity

I’ll be real here: no one irl cares, we’re so careless with our language people understand what you mean more by vibes than vocabulary. I’ve not heard a single person take umbrage with female outside of the internet.

Context is important. In your case, you're using female to designate a biological characteristic - aka, a reproductive ability that the cult is specifically exploiting. Some men, especially online, use "female" in a way where normally someone would use the term "woman", in an attempt to dehumanize women. It's unfortunate that the person you were talking to couldn't tell the difference, but as for never forgiving them, maybe a gentle correction could be more warranted?

Clyden!!!

Sorry but “woman” is not an adjective and its use that way is grating. You wouldn’t say “man teacher” and it sounds wrong. So does “woman lawyer” or “woman voter.”

The neckbeard/incel thing is using “female” when “woman” would be acceptable and more common, like “look at these females” or something. It doesn’t mean we have to abolish the word “female” entirely from the lexicon.

My personal rule is female adjective is okay, female noun is not.

Best be safe and call them "girl lawyer"

Yes you're over thinking this. A woman lawyer is just a lawyer. Same how a male lawyer is just a lawyer. Unless the gender of the person is important, leave it out of their job title. Use the word actor to describe both men and women who act. Flight attendant for men and women, or receptionist, or any other word. The vast majority of time you can leave their gender out of the description and it's fine.

That may be but if they would shorten "female soldier", wouldn't they refer to her as "a soldier" and not "a female"?

"Female" is fine as an adjective. It's when it's used as a noun that it becomes a problem. But that's not a new rule or anything.

Consider when someone says "Black people" vs "the Blacks".

Soldiers have a lot more than that drilled into them. Obeying authority, pushing down your own emotions, lots of shit.

I would never be a soldier. A fighter if needed, but not joining any traditional military. It will kill your soul.

Yes that's the issue, if you use them inconsistently. Males, females. Men, women. Same for boys, girls actually. Saying boys and women or men and girls can seem belittling to the other.

Fun use of "boys" has been really popular for quite some time. Me and the boys. Boys will be boys.