It's complicated. Marital satisfaction and happiness change throughout the 'family cycle.' Newlyweds are pretty happy. Having a kid is the lowest point. Having adolescent kids is better than infants, and not as bad as commonly thought. Couples who have sent their kids out into the world experience rising happiness, near newlywed levels.

But yes, those first few years after birthing a new child are definitely the worst. So in that sense, parenthood makes you less happy. But it doesn't last. As the kids grow up, happiness goes back up again.

BTW, the best solution to the 'marriage sucks after having a kid' problem seems to be this: change your expectations. :)