Marisa Mayer was a law professor and Karl's second wife. Karl and Marisa got married and had a son named Evan. Six years later, however, Marisa was too overwhelmed with the stresses of motherhood and left her family.
She met Susan at Lamaze class while Susan was pregnant. Marisa was pregnant with her first child. Julie knew that she was married to him and pregnant before Susan even knew about Marisa's existence. ("Opening Doors")
Karl tells Susan that Marisa left him because she couldn't handle being a mother.