What’s the best romantic relationship in a sci-fi story?

Not all science fiction is big on character development, and that's our bag too, but for this week let's talk about the best romantic relationships in a sci-fi book, short story, or series? Many of the titles we've talked about in the past have focused on solo characters, or military men and their machines, but some tales have an exciting romance to them, even if they don't work out in the end.

So what do you think the best romantic relationship or dynamic is that you've ever read in sci-fi? Maybe it's sappy, maybe it's two really well-developed characters, maybe there's a really great particular scene? Nominate some couples and the story titles below, and tell us in the comments why you chose that one.