I visited MK a couple months ago and actually witnessed a proposal!
I was standing in my favorite spot for Wishes - on the righthand side of the castle, on the way to Tomorrowland, there is a photo spot by Minnie's Rose Garden. It is a popular spot to stop and have a photo-op in front of the castle at night.
I was standing just to the right side of that, on the way down to the moat that separates the castle from Tomorrowland (it's where Tink flies over during Wishes).
Anyway, Wishes is well underway but every once in a while a few people would go up to have their pic taken. Well...
Just when the section of Wishes that features all the "Princess" songs begins, this guy practically drags his girlfriend up for a pic. I could tell she had no idea but immediately I thought, "O.M.G. He's going to propose!" No guy would be that jazzed to get his pic taken in front of the castle with firework hearts in the background.
Sure enough, he went down on one knee, presented a ring box, and said a few words to her. She had her hands up to her mouth and kept looking nervously around but he took her hand and got her attention and she said yes! They kissed, and now whenever I hear "So This Is Love" I think of them. LOL!
It was the single most "Disney" thing I've ever seen at the parks. So cute.