A common proof of appropriation that I have seen is that slash fic is often unrealistic ("gay sex doesn't work that way!", "real gays wouldn't say/do that!", etc.). This is an unfair judgment. Not only because it ignores the motives behind the writing style, but because it assumes the hetero default, and may be forcing the wrong gender and sexual identity onto the author. Queer experience can vary greatly, and even when it doesn't, authors can have equally valid reasons to want to explore, or avoid, a particular experience. For example, hospitals and St Mungos are not uncommonly visited in Snarry fics, but whether it is Snape or Harry who is the patient, the other rarely has any trouble visiting or making health care decisions. This could reveal a straight fan's lack of awareness or lack of concern about the discrimination queers face in hospitals. But it just as easily could be something a queer person would write....
For the past couple of years, Bugland and I have been in and out of hospitals. Because of my transsexuality, we have navigated the same hospital settings being perceived as a queer couple, and at others times as a straight couple. Despite anti-discrimination laws and being in a "blue state", we are still treated very differently depending on how we are seen. A close friend we met through fandom, and who is also queer, seemed surprised by our experiences. He has yet to be in the situation of being in a committed relationship, perceived as gay, and needing emergency care. If he wrote slash fic that involved a hospital visit, his lack of experience would likely be reflected in his writing, but that lack of experience in no way invalidates his queer identity. Likewise, Bugland and I may write Snape and Harry being treated no differently than a couple perceived as heterosexual, because we need an escape from the reality we frequently deal with.
When it comes to slash, I have come to prefer the assumption that the creator is making a valid expression of their own sexual identity, rather than appropriating gay culture, unless they explicitly reveal otherwise. A queer fan might slash because queer characters are sadly lacking in mainstream media and they want to see more characters like themselves; a straight female fan might slash because she wants to celebrate her own sexuality in a way that the mainstream media's "male gaze" denies her. While their reasons may seem different, I believe that both groups are essentially doing the same thing: responding to how their marginalisation is revealed in mainstream media, and accusations of appropriation based on generalisations about slash and its writers misses this point.
And now, to brave the vast snow drifts to buy poor bug more broth and jello and then hopefully to write a bit.