Connell’s decision to take Rachel to the dance instead of Marianne is selfish and unfair. He recently told Marianne that he loves her, but he’s
still unwilling to publicly acknowledge their relationship. As a result, it really
does look like he’s just “using” Marianne without caring about her feelings. But Connell doesn’t think of it this way, apparently thinking it’s no big deal for him to go to the Debs with Rachel, who—to make matters worse—is outwardly hostile toward Marianne. Of course, Lorraine recognizes just how insensitive her son’s behavior is, which is why she boldly renounces his decision, clearly trying to teach him that it’s not acceptable to treat people so carelessly.