I think we should accustom to strange sounding sentences. Maybe we can remember it easily, if it sounds weird or unnatural. And the learning is a bit funny too :)
I see one of the hints is "native" - if you say they are not natural - could you mean they are from another country? What would the context be for this sentence? Thank you.
The only context in which I have ever heard this sentence is in regard to breast implants.
"I am not natural" would be "Non sono naturale". Naturale is singular form, for masculine and feminine, while naturali is for either form of plural.