My husband asked at the end of the episode, "Why did both female leads get turned into vessels for evil higher beings?" This is a very good question.
Now, we all know that Joss fancies himself a Feminist...so what up, dude? I think it could be three things:
1) He hasn't really watched the show enough in the last two years to notice not just the similarity of plot lines but the similarity of sexes of the "vessels:" "Oops, you mean we already did that to Cordy? Huh. Let me look back over my notes. Oh yes, that was when Firefly was getting cancelled...I was a bit preoccupied at the time. C'est la vie!"
2) He's still just an idiot male and thinks that he's doing us a service by making the women suffer more. Because that proves that women are still oppressed...by the demon dimension...or evil...or something.
3) He really has his heart in the right place, but because the demon thing has already been done to death, and Jasmine was some sort of wacky rogue Power, but they still needed a really big Big Bad, they decided to pull out the Proto-Demon card. And Fred will prove in the end to be unconquerable and that women are equal in every way and don't need saving, just equal treatment... this sounds like I'm making excuses, doesn't it?
So really, I don't have an answer. I'm intrigued though. Why couldn't Wesley be the vessel? I mean, all that the Proto-Bad needed was to be inhaled so it could super-heat and liquify some guts--Wesley could have provided those quite handily. Does it have to be a woman because it inspires more pathos? Like all those cheesy movies "Lame November" or "Love Story" in which the male protagonists with hearts of steel melt like mac and cheese, and then watch the chick die in soft focus, leaving them "forever changed?" Is it as cheap a shot as that? It's more romantic to watch the broad keel over than any of the men? Or is it the old religious vessel, "Mother Mary Full of Baby" thing? I don't know, but I have to say...hmm.
Not that I didn't enjoy the hell out of the episode.
And did anyone notice that Wesley shot a guy?