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 clinical literature (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 superiority over the common masses. Almost invariably this takes the form of explanations about why nobody wants to have sex with them, and it all goes downhill from there.