Again, you're strawmanning me by presenting a misinterpretation. If a game wants to do sexism in the setting, it can. It's just not necessary, and there's no real reason(particularly in worlds with magic, the great equalizer) that it would favor men. It only becomes an issue when it is overwhelmingly common.
As for broodmothers; while you're right, it's possible they could make it so men are transformed into broodmothers, it also stretches believability that much more. And you're right, they aren't necessary to the story/setting; no more than an arch-demon being a dragon rather than a humanoid figure, certainly. If you've played Dragon Age and fought the Broodmother that you have to at one point, you'd know they're a bit more dangerous and thus a little more than simple reproducers. That said, yes, that argument could be made for them being a rape metaphor, though I think it's less trying to victimize women and more trying to say "HEY LOOK DARKSPAWN ARE EVIL DO YOU GET IT DO YOU SEE HOW EVIL THEY ARE?" and using rape-metaphors as a metaphor for that. Which is sexist for it's own reasons, but still. And I could very well agree that they are then turned into fetish fuel, with the tentacles and what not(though that could be a Watcher in the Water reference instead, too, or even just using the typical 'it's an abomination, so it has tentacles' approach)
As for why they exist, the only real response I can come up with is that darkspawn need to reproduce too, and making them do so in a less human way(that is, with broodmothers and desecration and corruption and parasitism etc.etc.etc.) was an attempt to further dehumanize the darkspawn and show how they're the bad guys. They could have done so in a different way, I suppose, by capturing people and transforming them straight to darkspawn somehow, but that stresses the plausibility of them being a constant overwhelming menace since they rely on each captured slave only being one darkspawn. I don't even want to think about them reproducing through the typical method of normal sexual intercourse; that's it own disgusting sort of fetish fuel, too.
Question: Where the fuck do trolls come from, again? Are they made from Qunari broodmothers or something?
NINJAEDIT:
To your first point: I disagree as to the reason they don't need broodfathers. Again, it's similar to a hive of insects; they can simply use the worker or warrior/drones to be impregnated. But yes, it could be possible to have men become them, too, one could argue.
I think some of the desire demons take different forms at some point, even to the viewpoint of the player, but yes, they are presenting them in the stereotypical succubus format. Why there aren't incubi versions I dunno. Only (poor) excuse I could think of them coming up with is designing the extra models or something.
It's not really a spoiler, and I don't think it's so much their resistance wearing off as it is that drinking the darkspawn blood and such is a taint in and of itself, and a poison. It takes it's toll on your body. But the fact that it happens, and they then go to the Deep Roads to die fighting darkspawn, tells me that it's unlikely a Gray Warden could be captured and then made a Brood Mother.
Although, that might actually explain why dwarves are more patriarchial; women warriors are more of a risk than male ones, since the men just die/make weapons if they're captured(they're dwarves, so it's still bad), while the women get turned into breeding machines. That's some fridge logic/horror for ya.