There is a polarity of body to consider when learning techniques. Men and women do have a different polarity, so in the application of techniques and that little spark that flashes brightly, or the dull action of a physical throw, there is a difference when men work with men or women work with women.
I do advocate people of same ability working the same sex, but not for the entire class. If , for nothing else, than to get a feel for the technique before moving on to a male or female partner. As for separation of training for male or female? Well, I guess that is up to the society or the people who are practicing, isn't it?
Even if they practice gender separation, sooner or later, they will advance in skill level, visit other countrys or dojos, and have to train without gender separation.
As for the polarity thing, you will need to study this when you see the hidden meanings for many techniques with pain, or in pressure point study by finding them within your Aikido techniques. But that is for another time.