One of the sweetest, most romantic films I've ever seen was Hotarubi no Mori e. It's a fairytale, of a sort, in which a little girl named Hotaru meets a friendly spirit Gin in the forest near her grandfather's home. The rule of his existence is that he cannot touch a living human, or he will disappear forever. As she grows and matures, their relationship evolves, but they struggle with the physical limitations his curse.
It all leads up to an emotional moment, when Gin instinctively keeps a boy from tripping. They both realize in that moment that this will be their only chance to experience that physical consummation of their feelings for each other, and they embrace for the first and last time. A truly bittersweet moment!