How does Bucky end his relationship with Marcia in the novel, Nemesis?

Bucky sees Marcia for the last time when she visits him in the hospital after he contracts polio. During her visit, Cantor says that he wishes to end their engagement, so that Marcia will not have to be married to a physically infirm man. Marcia protests, saying that she loves Cantor no matter what, but Cantor refuses to change his mind.