Hrothgar: Come to bed, my sweet. My kingdom needs an heir! I need a son! It's time you did your duty.
Wealthow: How can I ever lay with you, knowing you laid with her?
Hrothgar: I should never have told you. I should never have told you.
Wealthow, knowing that Grendel is the King's son, and thus who the mother is, refuses to sleep with the King.