On the DVD audio commentary, Schumacher said Poison Ivy should be played by the most woman beautiful in the world and Thurman fit the bill. I guess at the time, he didn't know about actresses like Jennifer Connelly, Alyssa Milano or Natasha Henstridge.
shareWell, Thurman's an incredibly attractive woman, and I think she fit the bill just fine. Beauty is always, to some extent, subjective, anyway, and there are lots of different "styles" of beauty that could be argued for that category (hence, Eris' nasty, golden apple).
Of course, Schumacher wasn't really the right judge, either...