I think in a lot of cases, things start out fairly even. Young couple, both pull their own weight, all is domestic bliss.
Then she gets pregnant and has a baby, total amount of chores goes up. Man does the same amount he did previously, woman picks up the slack because she's on maternity leave and has more time during the day to do it (and yes I know actually having more time is debatable, but if you're in the house more, you're physically more able to do it during those hours even if there's a baby hanging off your arm).
6 months or a year goes by, the new balance has embedded itself, and then the woman goes back to work, but the balance doesn't change. And voila, you've ended up with a lazy man who doesn't pull his weight, even if he wasn't like that at the start.