Men are from Earth...

Men are from Mars and Women are from Venus

a pretty common and popular phrase nowadays...
sounds quite ridiculous to me!

Just the idea of mating interplanetary species is absurd! (even metaphorically) What Mr. Gray (Author of the book) has done here is, he has made the situation even more complicated and eventually made ordinary people use comments like "I think he's from some other planet!" I'm pretty sure before the launch of the book nobody thought of men and women to be from different planets and saying something like somebody is from another planet existed only in the minds of Cosmology students.

As far as a relationships go I'm no veteran but even I've realized that men and women use, and strongly prefer, the same ways of comforting others – listening, sympathizing and giving thoughtful advice. If the inter-planetary theory is applied here, hitting rock bottom is almost inevitable.

So here's what I suggest...

"Men are from Earth, Women are from Earth... just deal with it!"

or

"Men have testosterone, Women have estrogen. So, there are bound to be differences and complications."

:P