For those who do, marital happiness stays, or actually increases if the couple has children. About 2/3 don't, however. They end up in relationships where the man works 8-10 hours a day, and the woman works 16 or more. Naturally, those wives (the 2/3 of married women) are not as happy--they experience a significant drop in marital satisfaction with the arrival of a new baby.
Even when both partners work, the woman usually does more housework, and more of the less desirable chores.
A few years ago Nat and I figured out (with some bad math) that she was doing 55% and I was doing 45% (this includes chores, work, and school). Naturally, we were both surprised at how low our percentages were. The only thing I'm sure of is focusing on 50-50 is not a good idea. Both partners need to be striving to help the relationship and each other.
How is the balance in your relationship? Are you focused on doing your share, or do you focus more on what you are doing and less on your partner?