Buy GW and change the setting steadily?
One option might be Tyranid forces developing a virus that attacks the Y chromosome, but it takes a while to build up. So men that have been infected for a while will die off due to the toxins, but younger boys will still be healthy. This would require a careful dusting of multiple worlds in the Imperium, so the virus has a chance to take root and become endemic.
Another option would be a virus setup that 'converts' Y chromosome critters into monsters (tyranid expendable bioforms). Similar to part of the backstory of 'Naked Earth', by Ferres - https://www.deviantart.com/ferres
You'd also need a way to convert women so they hardly ever age, they just get friskier over time.
It might be easier to just make your own universe
I was curious about it as well so I looked up the comic to see if there is any real reason for that.
As far as I understand it, people assume she is a slut because A. She hangs out with gamers club. B.Half her shirt is in on the outside and it's supposed to make her look as if she just got off.
C. She is very laid back, cheerful, very friendly to the point of clingy, and she is very interested in romances, what makes her very bashful.
D. On Valentine's day she seemengly snapped her fingers and suddenly got a boyfriend she made out in nurse's office. They didn't do the deed, but people assume they did since she was the one iniciating the kissing.
And last but not least. She is secretly very manipulative bitch. She will compliment two guys about something and make them fight just for lols and her own enjoyment.
As far as I see it, she is the most normal girl in the whole series. That girl is a player and a heart breaker, like most girls I have met. So either there is something wrong with me or other people.
comissar as a fellow guardsmen, i do point that, not only he was outnumbered(with to us is odd outside tyranidds), but also UNARMED i do say if he has not his laz rifle with him by his own fault , but in this situation his granade arm was restrained, and if he were to do his duty to the emperor your self, a valuable resource of the guard would be lost,due to range , the tactical choise was made ,punishment to him and maybe to me self due to my insubordination is still to ya autority, but as a combatent, his delegation of ofensive duty to ya his superior armed with a symbol of the emperors wrath, was still the most resource and security efective solution....what , we are reasigned to the front ? finaly, se ya later gotta sharpen my bayonett again...wait we are against what again here?