When K.J. Wiemer got married in the 1990s, he didn't think he was becoming a husband. Rather, he says, he saw himself as "one of two 20-something professionals living together, who were now entering the phase of 'officially' living together." Once in their 30s, however, and with two children, Mr. Wiemer and his wife realized that their roles had to change. "We sat down and decided together that I, as a lawyer, was likely to make more money," he says. "So I stayed working, and my wife stayed at home with the kids."

But the...