Do you mean to say, "I didn't quite understand what relevance the panda had"? In Ranma 1/2 there are cursed springs in China. Each spring had something different die in it at some point, long long ago. What spring you fall into determines what you turn into if you get wet with cold water - Ranma (the main character) fell into a spring that a pretty girl fell in to... Ranma's father fell into a spring that a panda fell in to. When wet with cold water, poof! Boy and man turn into girl and panda! (there were also small-black-piglet, cat, and duck that recall off the top of my head) Why would anyone go anywhere near these cursed springs, you might ask? Ranma's father's kind of a dolt, and takes his son there for martial arts training without fully understanding what the consequences would be for falling into water. (hot water turns them back to their "natural" form)
The only alternative for misunderstanding would be, why'd she tell Erwin to stay put. I expect she tested one elixir out on Erwin (or there was an unfortunate accident) and so of course he'd have to hide til they figured out how to fix things. Well, if sex turns you back to your natural form, then Erwin would just have to fuck someone. But who's going to be willing to test THAT out? Better for a human form to try the kinky stuff out first. And if she just doesn't bathe very often, it could be a while before they discover how to fix things... If you're asking, "Why sex?" I can only point out we're reading Yaoi. It has to be fit in there somewhere.
I was quite amused. She made a pretty hot man, too. I sort of have a thing for glasses.
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I didn't quite understand the ending..