The game is clearly making a divide between the player and the character they “control” through the game. It’s a mirroring of the big reveal of Undertale in a darker, more complicated way, like the rest of the game, thus why the main character rips out their heart and reject control the player had over them.
The fact the game is a demo that establishes this running theme of control and agency without full on payoff in said demo suggest this is going to be explored more when the story continues.
It’s not arguing that you have to be your assigned at birth gender, it’s arguing you cannot be this character in this other world (this series treats meta elements more like this is another world than this is a fake world, ala Metal Gear Solid 1’s forth wall breaks, to make you connect more with the characters and their struggles). In that lens, it’s making it clear the main character of Deltarune is not an extension of you, but his own person, and you’re supposed to question the full implications of that.
EDIT: I just wanted to add an apology. I didn’t mean to start anything or derail the thread. I wasn’t trying to question anyone’s feelings, just try to give some advice based on my own experiences with tangling my emotions with things I’ve hated or liked, but I clearly handled that badly.
Just going to drop this topic from here on, sorry.