I thought it was just a more scientific classification.

Scientifc classification by sex. Referring to others by their biological sex in a social context is weird and creepy. Even if you believe sex and gender are the same thing, it's still weird to call people by their sex. "Hello, male human. Want to ingest some fried pieces of cow flesh tonight?"

If you are calling a woman “a female”, and aren’t a cop discussing a victim or a doctor writing a chart, then yes, it’s fucking derogatory.

We’re not Ferengi.

I thought it was just a more scientific classification.

It's a classification of sex like biological characteristics, like chromosomes, hormones, and reproductive organs.  Are you asking about that?

Gender is a social construct. Just like race. Where you can be a Black person who is British. Or a Filipino American.

Using scientific terminology in colloquial speech is weird and creepy in most contexts. Calling kids "juveniles" and women "females" carries certain connotations, most of them bad.

Using female as a noun (rather than as an adjective, such as in the phrase "female firefighter", or any phrase of the format "female $noun") is generally overly clinical and dehumanizing. Some people do it out of habit due to their profession-- usually researchers or soldiers-- but they usually say "males and females", which while still weird isn't the worst.

The guys who say "men and females" are the ones you need to watch out for.

It's not inherently derogatory, but it does hold a connotation if you refer to women as females particularly in contexts where you wouldn't/don't refer to men as males.

Yes, using scientific terminology can be derogatory. But in this case, acting like the opposite sex is a species on its own, classifying them as animals and slurring all women as hoes gave it away for me.