Being born outside of the country to two US Citizens does not make you NOT a natural born citizen (and that is regarding his father even, not even applying to Mitt!)
The issue with Obama being born outside the US is that his mother wasn’t old enough to transfer citizenship AND his father was not a US citizen.
Mitt IS a NBC.
Mitt Romney may not be a natural born citizen if George Romney was born in Chihuahua, Mexico with natural born Mexican citizenship and/or U.S. citizenship, natural born or not. Natural born citizenship requires no natural born allegiance to a foreign sovereign in competition with any allgiances to the domestic sovereign.