Adding to what Matt says, given the length of time of getting a degree, the culture of post-docs in the sciences, and the length of time to getting tenure, if a woman in academia wants to have a child she's going to have to do it somewhere in the middle of that. No matter how enlightened of a spouse you have the physical demands of pregnancy/childbirth/early care probably are going to largely take you out of the game for 6 months no matter how much you try to get back to work. The game is set up with expectations of uninterrupted attention to research. Having kids interrupts that, but the only other option is not having kids.
In addition, unless I'm missing something from a quick glance at the study there's no way to disentangle whether having children has actually reduced women's work productivity or whether it's just made their peers more negatively disposed to them. During my time in academia, the sexism I saw largely manifested itself as bias against women who have children during their pre-tenure years. Women are fine, but women who waste time having children are unserious about their careers. I saw this coming from both male and female senior faculty members.
