Deltarune Chapter 5 has anal sex innuendos. Toby doesn't use the word "insert" casually...you know how he is with wordplay. Side B is the back of a person, where the anus is. It just means "When Ch. 7 comes, Toby @tobyfox will insert himself into the player's backside."

Lotta people feel entitled to deltarune's story and that "it has to go this way and reach these conclusions because that's what i want!" as if the game isn't literally about TOBY'S dream