Guys, just stop surprising women with public marriage proposals. It's a bad idea. No good can come of it. And what happens if you misjudged and she says no? Well, to make that possibility a little more real for you, here's an real-life example of how badly this could turn out for you. (Dude-in-this-video, I'm sorry to drag this out for you again, but it's for a good cause.)
I don't understand why people automatically assume that all public proposals should be accepted. Listen to that crowd; they LOVE this guy's proposal, and every one of them wants her to say "yes", without any of them knowing a single thing about their relationship, how long they've been dating, or even whether he would be a good mate for her (or vice versa)!
Imagine for a moment that you're at a coffee-shop. You turn to the guy at the next table over, and you say to him "Hey. Dude. Do you think that couple at the table by the window should get married?" His first reaction will probably be to look at you cross-eyed, but if you persist, he might spare them a glance and say, testily, "I've got no idea. Their relationship's none of my business," and go back to reading his Kindle.
...but as soon as the guy in that same couple you just pointed to goes down on his knee and pulls out a ring, everyone thinks that marrying him is a great idea, and that the woman should pledge the rest of her life to him on the spot.
It sets a bad precedent to set for the rest of your marriage, at the very moment of your marriage's "conception", to threaten your spouse with being made a social pariah if she doesn't agree on the spot.
Just don't do it. Seriously, guys. Don't.