Mama's Boys and Other Parent-Child Relationships

Another blog asks the question, "Why does our culture undervalue mother-son relationships?" Mother-daughter bonding and father-son bonding are seen as normal, natural and necessary. There are father-daughter events. But for the son close to his mom? The one cultural example other than "mama's boy" that I can think of is Oedipus Rex. I vaguely remember one - possibly two - times that there was a mother-son event. Personally, I grew up having and still have a close relationship with my mom (and dad, for that matter) and I know others here have as well - all without being "mama's boys". Other than culture telling sons that they have to be "manly men", why are mother-son relationships held lower in our culture than other parent-child relationships?