Why does Becky get so angry at Tom after he suggests that they “get engaged”?

Becky gets angry when she learns that Tom has been engaged before..... to Amy Lawrence.

“It’s so nice. I never heard of it before.”

“Oh, it’s ever so gay! Why, me and Amy Lawrence—”

The big eyes told Tom his blunder and he stopped, confused.

“Oh, Tom! Then I ain’t the first you’ve ever been engaged to!”

The child began to cry.