I agree with kiko, won't be able to sleep tonight.
You really have to be very, very crazy to create something like this.... not you, Aussette, but the people in EA who come up with it!
Yes, I really got the feeling now that EA is doing this on purpose. Back in The Sims 2 I've loved to crossbreed al those weirdos together to get a alienzompierwherewolveplantsim wizard and that was very awesome already. Especially because plantsims do relie on daylight, vampires on night, so together they can stand everything.
Still this is possible in The Sims 3 however, but the question is how. Letting them breed while being somewhere else in town seems to work, although I personally have not experienced this yet in my family.