Working hours, not income!
I earn much more than my wife but we share the housework 50/50. Why? Because she works equally long hours and is just as tired at the end of the day than I am. I make the house dirty (arguably more than she does as I work from home more often), wear clothes and eat - so why shouldn't I do my fair share of the work?
To me the argument that one of you earns more and therefore deserves to do less of the homework is flawed (but is probably sustained by the men who are earning more for the same job than the woman in their life - still a regrettably common situation).
