"It isn't that.

It isn't even because she's afraid he'd be a dissipated husband and she wants to be safe.

It isn't because she's religious or hates wildness; it isn't even because she hates wildness in him." "Well, but look how she's thrown him over for it." "No, that wasn't her reason," said the wise Mrs.Henry Franklin Foster.
"If men only knew it--and it's a good thing they don't--a woman doesn't really care much about whether a man's wild or not, if it doesn't affect herself, and Isabel Amberson doesn't care a thing!" "Mrs.Foster!" "No, she doesn't.

What she minds is his making a clown of himself in her front yard! It made her think he didn't care much about her.

She's probably mistaken, but that's what she thinks, and it's too late for her to think anything else now, because she's going to be married right away--the invitations will be out next week.