Christine Whelan thinks the main point is that smart women marry at the same rate as other women, just later in life. This is an important point, and probably true or nearly true. The deeper issue than whether smart women will marry, though, is whether they can get all the education their brains can use, and launch their high-achieving careers, and still have kids at the same rate as other women.
I think smart men are coming to see that women who want lots of education and high-powered careers can also have kids – and given the right supportive husband, they will have great kids.
Whelan approaches marriage as being primarily about the man and the woman. The deep sociobiological, social, and, for most couples, personal impetus for marriage, though, is to have and raise children. There have always been highly educated and high-achieving women. Until recently, though, that path did present very high obstacles to having and raising children. The women knew it, and the men they might marry feared it. One solution has been happy childless marriages of high achieving partners. These are fine marriages, and I am not in any way disparaging them. But most men, and especially most women, do wish to have a life with children, integrated with all their other work.
Whelan notes in passing what others who have studied high-achieving mothers make central to their research (especially if the researchers are themselves high-achieving moms): you need lots of help. Nearly all mothers who successfully raise children have helpful husbands, or the functional equivalent. Many of the women Hewlett interviewed at the highest levels of their careers bluntly say that in addition to a supportive husband they needed to "hire a wife." This seems to me a reasonable solution, and being that nanny is as honorable a kind of work as the professional careers she enables.
The deeper point of why smart men marry smart women more than they used to is that both of them realize that they can still have their smart careers and have smart children.