I would subscribe to the idea they just do what Dante does and tuck it into their armour. They don't actually have 'big' hair in that it's more just long and flat and it'd mostly likely just sit flush against your helmet without much issue.

Please explain than, how this guy will tuck his hair in, asap, if sudden bio-virus atack will be
spread

Or in the middle of combat

I didn't say it was a fast thing and besides sometimes we gotta flow free like Daddy Sanguinius. Also 40k has the thing of a lot of named units don't actually ever wear helmets (Realism? For who? For you, maybe, for us noooooooo.)

So it's the thing of assuming they use the Dante method for him and helmeted mooks and otherwise assume they just go helmetless.

So it seems Games Workshop just missed the realism point with long hairs. Im satisfied with the conclusion of this glorious discussion

IF you had a glorious mane like that, would you have a helmet cover it? Or would you let its beautifulness protect you. Also if you read the recent short story it says the wolf scout has a beard wild hairstyle etc... nd he keeps his helmet with him but would never wear unless they need to in case of survival

Well everybody uses helmets - pretend its sudden hull breach in cruiser. You must obviously put the helm asap or you will die. And in this case such hair will be cause of death..So we are missing some arcane tech or GW for sure didnt thought about it and made a breach in logic.

no one understands how their barber works

Well yes it's quite troublesome to be so fabulous in time for unexpected visitors. Heaven forbid you came to battle without your thrice blessed "face of the emperor" hand mirror. Fortunately daily hours of conditioning and rinsing makes one's locks flow luxuriously into the helm with a few mere moments by a skilled hand. You have to know these things as the legacy of an angel, you know.

The Primarchs don't need helmets in vacuum.

And not all BA have super long hair. Most of them have shaved heads or pageboy cuts at the least.

SW though, I am pretty sure have holes in their helms that they slip their braids through or something.