It's not a hentai scene. It's the remake of the scene where you try to get her panties, I guess.
You sort of need it in order to get more information. At least, that's what I remember from Rance I. Miki isn't really plot relevant in 01, and so you'll only be seeing her in one event. She has no h-scene, since she'll blow you up. Kentarou doesn't show up in I as far as I'm concerned.
If you want to H Miki without a game over, go rape her in Kichikuou. Be prepared for the consequences though...
I am amazed at how logic works in the games. It's like they pull it out of nowhere. Like how am I supposed to know that this is here!? Anyway, that's the thrill of the game. If you ever hit a wall, there's always a walkthrough.