What does Hamlet mean when he says the "heyday in the blood is tame"?

This quote is from Act III, Scene 4 of Hamlet. In this scene, Hamlet confronts his mother about her marriage to Claudius. Comparing images of King Hamlet and Claudius, and pointing out that Claudius is worthless by comparison, he asks what could have driven Gertrude to marry the man, especially so soon after her husband's death. Perplexed, he says that

You cannot call it love; for at your age the heyday in the blood is tame, it's humble, and waits upon the judgment; and what judgment would step from this to this? 

What Hamlet means is that Gertrude could not have married Claudius out of love, because at her age, sexual desire ("the heyday in the blood") has gone away, "humble" with advancing age. So because Claudius is nowhere near the man King Hamlet was, and Gertrude's sex drive should be gone anyway, there must be some other reason why she chose to marry this man, who, Hamlet reveals to his mother, is guilty of the murder of his father. This is one of the most intense scenes in all of Shakespeare, punctuated by Hamlet's accidental murder of Polonius, who is hiding behind a curtain. Hamlet's questioning of his mother both shocks and horrifies her, and sets the stage for the unfolding tragedy to come.