This is what I said to someone who asked a very similar question about the same thing a while back:
'Females' is, effectively, a 'technical term' you might say, that isn't used in normal conversation. It's used specifically in situations where distance from the subject being discussed is intentional. It is the sort of language used in police reports, medical reports and the like...when it's even being applied to humans at all. Its use is perhaps more common referring to animals; it's the sort of terminology you'd expect to hear in a nature documentary.
The people trying to push its use are intending to make the subjects - women - sound 'other' and separate and alien by referring to them as 'females'. Not everyone who is picking up this terminology intends it that way, but the connotations are unavoidable because of how language works in common use, and therefore if you don't intend it that way, you badly need to be made aware of it so you can stop.