Fun- there is ALWAYS something "open" in any position you are in. Otherwise self defense would be quickly summarized by "assume that position where you cannot be hurt and stay there"
But in response, no matter which leg is forward, the other side is more open to a kick. Only the fact that Aikido usually has really structured ways of standing when attacking and being attacked lead people to think that they are "more closed" one way than the other.
Actually, the "groin kick" question kind of demonstrates something about our stance in Aikido... I think that our stance is a good deal longer (and somewhat wider) than most simply -because- we don't have to worry about below-the-waist strikes. For other styles, they have to keep their legs much closer together. That way, it takes less time to shift all their weight to one leg so that they can lift the other one into position to block an incoming kick.