The never-ending battle against woke Hollywood and SJW infused entertainment media [trigger warning]

Technically, GW is not wrong...
As stated in the most precise and complete source regarding Custodes (the Black Book series of the Horus Heresy), while space marines are created using male geneseed (the Emperor / primarchs), custodes are made using mysterious techniques that go back to the dark age of technology, involving profound genetic alterations of a very young subject.
So, it is indeed plausible the existence of female custodes.
The fact that is wrong, is the kinda dismissive attitude adopted by GW to insert this... "of course they were!"
40k lore doesn't work that way, within canon there are plenty of vague references, contradictions, unreliable sources... for a thing to be sure, if must have been reiterated at least a tenth of times through different sources (books, codexes and so on).

Female custodes is not wrong per se, but the way GW handled this is completely stupid (and dangerous for any future change they would like to introduce to the setting).
If you undermine too much the very foundation of your narrative universe, the risk is to lose the fan base (see Doctor Who)

 

yep, it was in a previous codex and indeed it doesn't say anything about "male only", but as you can see it's nonetheless pretty vague. And the fact that up til now the custodes were organized into "brotherhood" was of no help to actual GW's stance.
The one and single explicit recerences to female custodes was made yesterday, in the new codex.

Prior to that, there was only a passage in a book of the horus heresy (Siege of Terra), written a couple of years ago, when a primarch (Sanguinius, IIRC) was observing a spaceship with custodes in its proximity, and he refers to those as men and women, clad in golden armor.
Alas, the sight and the description was including not only custodes, but also the personel of the ship and the Sisters of Silence (which are women, in golden armor), so it's far from crystal clear.

 

Oh, i see.
Well, i know that sons and daughters is males and females, but in italy when you say only "sons" and you use it in a general context (for example, "we are all sons of God") it includes both genders.

Looking at the text you posted, i was thinking it was the same.