I see Steve here has been forced into defending against all comments on SF love stories:
In the one story really stands out: The Left Hand Of Darkness by Ursula Le Guin
If there is a better novel dealing with the emotions of attachment in the SF cannon, I can't think of it. Also it is pure, down the line, ideas based SF with a full explanation of the world it is set on... I don't think there is a better example in the genre.
On a second front, my running obsession with Sturgeon pops up again with this one... Anyone with a passing knowledge of SF has come across the quote, oft repeated, that "all my fiction is about love"... Now admittedly he didn't write enough novels for one to fit the bill (although it's possible that Some Of Your Blood is a very wierd love story) but his short stories have this theme again and again and again... Love, love, love... All the way from Bianca's Hands to Slow Sculpture.
However, I can see where your coming from, and I think I know why it's the case as well. While love is a common subsidiary theme in SF (Is The God's Themselves by Asimov a love story? What about the gay romance central to OSC's Songmaster?) it is extremely rare for it to be the centre of the story. I suspect there are sound structural reasons for this - I remember reading an essay by Larry Niven explaining why he found The Patchwork Girl and his Gil Hamilton stories extremely hard to write (he mixed SF and Detective Fiction in it). When you start reading a Detective story there are some strong expectations about how the narrative will develop and where clues will be left that should allow the reader to know at the end of the story how it all fitted together. Whereas a good SF story is built around the idea, and therefor this "reveal" moment at the end of a detective story is particularly jarring when the story should be built around how the idea interacts with the setting... Or to put it another way:
Q: How did the victim die in this locked box story? A: Oh, someone has invented teleporters!
I'm wondering if the same problems occur with love stories (I haven't read enough romance to know how the structure of these stories is constructed). I'm guessing that love stories have got to have an emphasis above all else on the two characters, and an arc to their relationship... Wouldn't this jar heavily against the SF structure which asks either for a technological/science concept for the whole thing to hang on, or on the adventure/quest structure that is needed in spec-fic?
I bet it can be done tho, there are masses of really good detective SF stories, and I still think they are a fundamentally bad idea.