To start things off, there are two temptations to be avoided:
First, it should not be a question of whether Joss Whedon - the mere human person who shits in a toilet - ' is actually a feminist' or simply lusts after kung-fu girls. That reduces everything to a futile search for secret clues about an old dude's ability to maintain an erection, and has nothing to do with feminism. Instead of fighting sexism, that is only fighting 'a sexist' (who may not even exist). It's a waste of everyone's time. Second, it should not be a question of the kung-fu girls themselves should exist in media or not. That puts all the emphasis on the rudimentary plot content and distracts everyone from the actual ideological critique. And to do so is, again, not feminist. The kung-fu girls already exist, they're not going to unexist, and they're a fact to be interpreted.
The actual question is what sort of feminism is being promoted, unwittingly or not. And it's just a very basic type of American-style liberal feminism.
The Whedon ideology is easily summarized by this 'campaign ad' where he jokingly claims Mitt Romney will bring about a zombie apocalypse. This mustn't be dismissed as meaningless lolrandomness. Whedon is, after all, a professional writer of 'politically charged' genre fiction.
Pay attention to the sequence of the subtle rhetorical shifts: Whedon warns of the threat posed by dangerous subhuman poors, then alleges that Romney is bad because he "already" sees the poor as dangerous subhumans, then says that Romney and his followers are themselves a bunch of dangerous subhumans.
This all amounts to a plea for liberal tolerance. Whedon boasts that he tolerates the poor single mothers, while Romney does not. But Whedon fears that these poor single mothers will pass a certain threshold and become intolerably poor. They'll become violent, perhaps terrorists. They'll riot. This is why he makes the final shift and says Conservatives are the true subhuman: because their intolerance is the ultimate intolerable behaviour - the greatest threat to the smooth functioning of the system. Unless we tolerate poors today, placate them with charity, they will rise up and murder us tomorrow.
Note how the point about reproductive rights is linked to the nightmare scenario of a woman being attacked by her own child - both of them made into the abject homeless who are a burden on us all. 'Feminist' Whedon is, first, unable to recognize that poor women already are reduced to intolerable, disgusting, inhumans every day as an effect of poverty. Then, he is not able to reduce himself to a fellow zombie out of solidarity. For a guy who wrote like a thousand hours of vampire drama, he misses the point badly:
"It is clear that in classical Hollywood, the couple of vampires and zombies designates class struggle. Vampires are rich, they live among us. Zombies are the poor, living dead, ugly, stupid, attacking from outside." -Zizek
Whedon's feminism has certain unspoken limitations. It does not extend to 'intolerable' women - those who are ugly and stupid, belonging to the outgroup... Think of the zombie girl in Cabin In The Woods, who is thoughtlessly killed. Think of the army of zombie drones from the future, that threaten the Earth in Avengers. They get nuked.