Fascinating thread. Could have written OP myself Pruners and your subsequent posts.
I don't know many happy couples who are "equal" in the accepted sense of the word.
Generally speaking, I don't feel equal to dh in terms of intellect, earning power, or personality. But agree that equality can shift over time, in certain contexts, and according to different situations.
For example as an an expat 'trailing spouse', my dependence on dh has been depressing at times, but this has lessened dramatically in line with my ability to speak the local languages.
We have recently redefined our areas of responsibility and wierdly, I am happier now that my dh is less involved in the housework, childcare than he used to be. I have my own 'sphere' of power/control in the home iyswim. Before, both of us used to try to share everything and it was stressful and confusing.
I too deeply resent that he can walk out the door to work when he wants without giving a thought to childcare arrangements. I only work 21 hrs a week but have to have everything set in stone to do that.
Sorry for the digression but it's more than just the practical issue, I envy him his ability to be completely focused and single-minded about his work to the exclusion of everthing else, something that I have consistently failed to achieve since giving birth.
So in answer to your original question, yes, and rather depressingly, perhaps it is all just a matter of brain chemistry ...
Over to the evolutionary anthropologists ...