Why does Becky get so angry with Tom after he suggest that they get "engaged"?

Tom asks Becky if she would like to be engaged to him; his definition of engagement is simply telling "a boy you won't ever have anybody but him" and then sealing it with a kiss. After whispering, "I love you" in each other's ears, the bashful Becky and Tom kiss. Inadvertently in his giddiness, Tom blunders that he was previously "engaged" to Amy Lawrence. After learning this, Becky rejects Tom and breaks into tears despite Tom's pleading.