I don't know what would make anyone think "a lot" of people here are anime obsessed. I've seen a few people with anime avatars and a handful of us talk about trans anime on the TV board, but it's a significant minority, as far as I can tell. Maybe there's a secret trans anime-fan underbelly I have yet to meet.
I'm an fan, but I'm certainly not obsessed. I started out watching 2nd gen anime with lesbian and gender-bending themes (Sailor Moon has Sailor Uranus, who dresses and presents as a man (later, a villain named FishEye who presents as female, and three men who magically transform into women); Revolutionary Girl Utena has a girl who wants to be a "prince" and another girl who is a pants-wearing lesbian).
I think the nature of anime to address more complicated themes has attracted gender deviants to it.
Now there are even a few anime that have trans characters in them. It makes sense to me- when was the last time you saw a western cartoon about a transsexual? I'm not the type that thinks japanese animation is "better" than western animation (some are way good, some are so bad it hurts!), but because of the history of the country and the medium, animation has a different role in Japan, and the things that make it different allow for the formation of a niche group of fans.