In my mathematical writing in grad school there used to be sentences like "Whenever x is a fish, then x is an animal." (Yes, that's what my dissertation was on!) I am a native speaker of English and this seemed fine to me.
My adviser (who is not a native speaker of English, though it's not entirely relevant) insisted that this was incorrect, so I stopped using it because he was pretty much the only one reading what I wrote. He suggested replacing "then" with "we must have that" (which I did).
I still wonder, though...is it really incorrect?