Just trying to put myself in the shoes of the anti-female-space-marine crowd...
Space Marines have always been all men. I don't remember if there was ever an in-universe reason for it although I'd bet there is. Suddenly having female marines and pretending that it's always been that way would be kinda dissonant. It'd be like suddenly giving Gordon Freeman full voice-acting; not directly contradicting anything, but against pretty long-established rules. The alternative would be advancing the universe of 40k in order to introduce a reason for female marines to exist, and I understand GW would rather self-destruct than advance the plot of 40k.
Either retcon or new canon, there's a problem: because it's about gender, it will feel contrived at this stage of the game. It'll feel like a choice made so that the game could stay relevant, rather than because whoever writes 40k lore genuinely decided to add it. I wouldn't call it pandering, but it feels artificial and game-y. Like breaking the 4th wall on some weird meta level.
I don't give a damn about the lore of 40k, I just like the memes and videogames. I think female Space Marines would be cool too, all of what Dunamisdeos says applies. At the same time though I can understand why it would rub /tg/ grognards who've been playing tabletop 40k since the 80's the wrong way. I have no idea if what I said above is what they're actually saying, I just tried to put myself in their shoes.