Are you sure? I just finished replaying his route just to double-check if any of the answers that I gave were incorrect and they all appear to be correct (you know they're correct when you see the stars appear after you've chosen an answer)

I also verified with another friend of mine who has the game and she also followed the answers that I have provided and ended up with the "Happy Wedding"

When you and Keito meet at the restaurant, he pulls out a small white paper bag which he bought with the money he earned from the construction site. You'll find a small box inside a small case which is a ring that he proposes to you with. Otherwise, you would be getting a pair of earring instead.

I really don't know why you're getting the normal ending instead of the adorable ending that Keito has ^^;; Could you try again and see what you get?