- Yes, the Sisters of Battle exist. No, they have nothing to do with this question. (And don't get me started on the Sister of Battle harem girl model; would one beefy, oiled Space Marine in a loincloth kneeling at a Dark Eldar's feet be too much to ask?)
- Yes, I know there were originally female Space Marines (check out the left column of the ad below -- even though they're not called Space Marines they clearly are. And yes, I know that one of them is wearing power-armour chaps).
- Yes, GMs in individual campaigns are free to make up whatever they want; the bizarre thing is that so few people seem to think this.
With those anticipated points out of the way, let's take a look at the discussion.
Now, I'm sure that there's been a lot of writing about this I haven't seen, but the argument in favour of there being female Space Marines is simple and obvious: it would be fun to have opportunities for both male and female characters in this group, and it would be nice in general to see some female characters in the 40K universe, which has ... few.
(I'm speaking here, of course, about people who care about the "lore" or "fluff" of the setting; to many players, these are just figures on a table, whose genders or personalities are irrelevant.)
There have been a number of different responses, but the only interesting one that I've seen so far is that it would be thematically inappropriate for there to be female Space Marines, because Space Marines are a dark parody of masculinity. Huge, hyper-muscular and either stoic or brutal, they are what men would be like if they had no stereotypically "feminine" traits at all. Having women among their ranks would spoil that image. Further, this argument goes, they're not meant to be admirable -- they're the ruthless enforcers of a monstrous tyranny, dripping with imagery from the worst regimes in history. "Women, too, can be Space Fascists" isn't a message positive enough to mess with your canon to send.
And I think that argument has several good points. There is certainly art and fiction that presents the Space Marines as grotesque hypermasculine-but-asexual creatures, and certainly introducing a female character who was in any way different from her male counterparts might tend to push against that theme.
But ... and here's where I think this argument is incomplete ... it wouldn't mess with that theme nearly as much as the bazillions of words of fiction in which Space Marines are presented as noble, heroic warriors, maybe a little gruff and unsympathetic but ultimately good guys whose virtues of discipline, courage, loyalty and self-sacrifice are supposed to be admired. I'm not even talking about people who view playing Space Marine or Space-Marine-like characters as an excuse to be a tiresome, hard-charging, no-emotions-but-duty-and-wrath jerkoff, I'm talking about Marines presented as no less terrifying or unlikeable than the protagonists of a sepia shooter.
And ultimately that's the problem(?) with a corporate franchise like this one -- it consists of a huge variety of art, fiction, models, games and so on, all created by different people and all necessarily having different thematic and artistic approaches. That can be as obvious as a world that's supposed to contain both Shadows over Bogenhafen and Blood Bowl, or as subtle as the difference in portrayal of the rule of Chaos in the setting's cosmology between the work of different writers. And it's not just limited to Warhammer, of course -- both the avuncular boy-scout Batman and the grim avenger of the night are Batman and always will be.
So if Space Marines were always portrayed in a way that meant that "a grotesque exaggeration of typically masculine qualities" was central to their thematic role in the story, then it would be true that having female Marine characters wouldn't make a lot of sense (although you could still tell some interesting stories with that concept, couldn't you?). And furthermore, players of those games would probably be less likely to be interested in playing female characters. But they're not always portrayed that way -- they're portrayed in a wide range of different ways; in the original version of the game, they're clearly just hard-bitten Special Forces types with some weird rituals, in much of the second edition they're portrayed as basically superheroes, and in the Horus Heresy books they're often presented as quite flawed and human despite their superhuman powers. You just can't make that argument consistently across the franchise as a whole, although it's a very valid interpretation in itself.
Don't like a female Marine miniature? Don't put one in your army; problem solved. Want to include female Marines in a Deathwatch game (which may even be suggested in the book, which I haven't read)? Go for it. Sadly, this probably doesn't extend to GW models, since they seldom make female humans, let alone Space Marines, and that seems unlikely to change, but I bet if you have a bit of a look around you can find some suitable conversion heads or Marine-alike models.
And this diversity of possible interpretations is a feature, not a bug. You're never going to nail a multi-creator, decades-long work down to a single interpretation, and that's OK. The inconsistencies and contradictions and changes create opportunities for varying recombinations of the source material, which is fun as heck. That's what Warhammer Sorta Thousand is all about.