Well, women are not completely absent from The Road, although the female characters are temporary and tragic. There is the argument flashback between the father and mother, where the mother has decided to take her life rather than face the reality of the post-apocalypse world. Her decision, and her absence from the book is a major theme: the absence of hope, the absence of future. Most of the other women in the book are horrible victims, those held captive and bred for food, which simply reinforces McCarthy's theme of hopelessness.
Only at the very end of the story does he give a glimpse past the hopelessness, with the woman who embraces the orphaned boy.