So a couple of days ago in class we discussed Milkman's reaction to Lena's rant.  There were a lot of important points made, and I feel that they were all relevant.  But I think it is also important to consider that Milkman doesn't think very well of women.  All his life, the only source of discipline and power has been his father.  His mother is very submissive.  She herself says that she is a mentally and spiritually small person, who depended on her father and then on her husband her entire life.  She doted on Milkman, but Milkman never really saw her as a person.  Ruth never once stood up to Macon dead to defend herself or her daughters.  Magdalena and First Corinthians are also very subdued women. They never made Milkman do anything for himself, and they were constantly aware of Macon's power over them.  We the readers didn't even really see them as characters with personalities until later in the book, when F C meets Henry Porter, and Lena has her rant.
Aside from his immediate female family, Milkman knows very few other women while growing up.  His mother has some female friends, but he doesn't interact with them.  He meets Pilate, who is arguably the most important and strongest woman in his life, and he doesn't treat her very well at all.  He tries to steal gold from her, and she has to bail him out of jail.  He treats her granddaughter badly too.  When he breaks up with Hagar, who truly loved him, he sends her a note because he thinks that it's the nice thing to do.  The nice thing to would be to break up with her in person, to her face.  Sending a break-up note in a letter is very rude and demeaning, and the fact that he thinks that it was a nice thing to do shows both how little he knows about other people and how badly he treats women.  Even when he considers getting married, he doesn't think about it because he loves the girl.  We never even learn the name of the red-headed girl he contemplates marrying.
Milkman doesn't think very highly of women.  So when Lena rants at him about how he never did anything in his life and never thought of anything but himself,  I think he goes into shock.  Suddenly Lena has done something that upsets all his preconceived ideas of what women are like.  Sure, Pilate was strong and could stand up for herself, but Pilate was a very manly, strong and tall woman.  Lena doing this just blows Milkman out of the water.