"Why must these women give in to a (purely psychological?) urge to become biological mothers?"
I don't understand it myself. I've been married 20 years and never had the desire for children, so I never had any. You should see the flack I've gotten on FR for my "selfish" decision! What posters fail to express is why it's any of their business. I guess if you don't do what the posters generally think is right in your own life, they'll let you have it, whether you have kids or not.