Currently not possible without save editing.
If you're on Windows, go to %appdata% > Godot > app_userdata > Love Maniac > save, then open meta.json.
Change the "side_bj" line from false to true under "unlocked_scenes" and save the file.
If you're on macOS or Linux, I don't know the paths for those, but I'd imagine it's the same, just replacing AppData with its macOS/Linux equivalent. On Android, it's impossible to edit the save.