Why does Macbeth tell his wife that they should only have male children?

Macbeth says this in Act I, Scene 7, after Lady Macbeth chastises him for his misgivings about murdering Duncan. He tells her:

Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males.

What he means, more or less, is that Lady Macbeth's spirit is characterized more by what he (and Shakespeare's audiences) associates with masculine virtues. These include courage, ruthlessness, and violence. Indeed, just after hearing of the witches' prophecy from her husband, Lady Macbeth resolves to "unsex" herself so she can become ruthless and pitiless in order to push her husband, who she views as lacking in courage and overly possessed with the "milk of human kindness," to murder Duncan. This would seem unnatural to Shakespeare's audiences, and is an example of what the witches said in the first scene: "What's fair is foul; what's foul is fair." In this passage, Macbeth says hise wife's traits should be passed on to sons, not daughters.