There's nothing more paradoxically trivial, yet central to society and human psychology than gender:

• We use it as the bedrock for our identity, yet half of humanity doesn't share it with us.
• In groups, we care so much about it that men and women effectively live in different cultures.
• We build elaborate stereotypes about the opposite sex, but they are often wrong and only exist because we fear rejection too much to just ask.

...so why gender-bending fiction?

1. Characters are at their most interesting when life challenges them and they are forced to react.
2. The easiest way to reveal someone's true nature is to challenge their worldview.
3. I know of no more basic way to challenge someone's worldview than to challenge their preconceptions about gender. (eg. identity crisis, culture shock, etc.)
4. In most fiction, gender-bending occurs in ways that characters never thought possible, which helps to unsettle the supporting cast too.

That's not to say that every story in this index is a gem. Some of them just use cheap one-off gags involving cross-dressing men. However, it's definitely a good start.