As amusing as this whole boy/girl fight is, i may as well put my input in. Personally, from what i understand, men were held in much higher esteem then women back in 'bible times'. Therefore, it makes more sense to me, that if you would want the image of God to be so mighty and powerful, you would not portray God as a lowly woman (just refering to how it was back then, i'm not having a stab at all you girly's out there).
If God were to have been portrayed in the image of a woman, it may have damped 'his' godly image, and thus, adherence to 'him' might be lessened, because being a she does not command as much social respect compared to if he was a man.
Now, some of you might argue that if God was so mighty and powerful, why would it matter if he was portrayed as a man or a woman. Well firstly, it helps give God a bit more of an identity, so as to help us comprehend just a bit better, the nature of God. As i said, 'he' helps to convey the identity of a man, and thus, at the top of the social ladder.
Secondly, why does gender matter if he's a God? Well why did the skin of the persecuted african-americans matter if they were still men, women and children like the rest of us? For that fact, why were black people all over the world looked down upon for the colour of their skin, not who they were?
For the same reason why God would not have been held quite as highly in their society if he was portrayed as a women. It was just easier to portray 'him' as a man, then try to get over the social hurdles by proclaiming him as a woman.
Thats just how i see it.