I can believe that that's true for a significant portion of humanity- that they would choose to have children even knowing it would be bad for their happiness in the long run. It isn't true for me, though, and there are large numbers of people for whom it isn't (or else childlessness in the West wouldn't have risen so much).
Having children fundamentally changes you, mentally. What may not have been a priority before, suddenly becomes a terminal value in itself once you bond with a little one. This is definitely something hard-wired into brains by evolution--ask any parent about their experience!
There is a lot of research on this topic. I know later studies contradict earlier studies, but I'm not sure who to believe here. Wondering if anyone can help.
In case it isn't clear, the basic question is the effect of having children on happiness.