You're absolutely right about everything you said, let me explain why I did it this way:
The game is short because it was designed from the start to be short. When I began working on it, I was just starting out in game development and wanted to release a few free games. And since I wasn’t going to charge for it and was also working on other games at the same time, I couldn’t afford to take too long making it. The difficulty is so high because it’s my way of extending the game. If you could complete it on your first try without dying, it would probably last only half as long as it does.
The game already explains who Melody and Onpu are, giving you scenes where their relationship develops, like when Melody cooks for her and plays the piano at her house. It also explains the background of the story—what they’re doing in that world, why they’re there, and what that world actually is. The game explains everything; adding more scenes would have just been filler. Filler that a lot of people would probably have enjoyed, but filler nonetheless.
As for the sex scenes, initially, this game wasn’t going to have any. I made it without them and released it that way, only including the final kiss, which you said you’re fine with. What’s the problem? No one downloaded it. It wasn’t until I added those sex scenes and tagged the game accordingly on itch.io that it started getting a lot more visits and downloads. So, if you want, you can pretend that canonically they don’t have sex that night and just kiss at the end of the game, and that’s it. After all, that’s how I originally intended it. But to reach a wider audience, I had to include those sex scenes even if they didn’t quite fit.
Lastly, thanks for saying you liked the concept and story.