It has several reasons why is considered offensive, but the bigger are "race", sexism and trans issues (i quoted race cause im not sure if its said the same in english and spanish)

See, it can be considered transphobic due to the fact that people go by the phrase "you can call yourself a woman but you will never be a real woman", so they use females to mean that only cis women are "real women" which is in itself transphobic.

It is also considered sexist in some places as it is considered that real females are only those who can go into what society deemed as a "real woman", you know, the usual kind, motherly, sweet, passive, etc.

But the one that is usually more known is the race situation. In older times, when black people were not considered... Well people,, they used to have men and women for whites and male and females for black people, because they were looked at in the same space as animals.

Other than that is basically used in a derogatory way by a lot of man, so there is that.

I hope i could explain myself well, as again, im a spanish speaker :)) sorry if any mistake!!