When you are in a public place and witness a marriage proposal, I'm sure that would send fluffy butterflies tingling all around your body. Personally, I have never seen one in my life; but I could imagine witnessing one. I've seen tons of videos on Youtube on proposals gone right.
I think that apart from being romantic, some guys do propose publicly so the girl won't have the courage to say "no".
That, for me, would be the most cringe-worthy moment ever.