Mention the subject of "love interest" in connection with the sf genre and you might get various plausibly dismissive reactions from those who do not know it very well. SF, they might say, is too concerned with other things; with sociological extrapolation or derring-do amongst far stars. Its sense of wonder is ill-fitted to the different kind of wonder, the personal nuances of a successful love story. Trying to put both kinds of greatness into a story would surely lead to overloaded plots and confusion of aim.
To which I reply: it all depends on what kind of sf story we are discussing. To begin with it should be admitted that here are stories which are such generally good portrayals of character and situation, that there is bound to be a well-handled love interest simply because the people in the story are human, living human lives. This is the case with Jack Vance's masterpiece, Araminta Station. Vance in sheer exhuberant creativity gives us the world Cadwal with its political stresses between Conservationists and their "progressive" opponents, and his hero suffers love and loss and love again, in the course of his schooling and career, which is all intimately linked with the fortunes of the society in which he lives. You could call Araminta Station a character-epic set on another world; its status as sf is real, but it also has something of the assurance of a "mainstream" novel of manners.
The love interest doesn't have to be as nuanced as this in order to be a successful part of a story. In Heinlein's The Puppet Masters, an alien-invasion thriller, the course of true love between hero and heroine is marred for a time when he falsely believes she has allowed herself to participate in a plot to manipulate him into undergoing a horrible test involving contact with a parasitic alien. The episode is a good example of a love story "subplot" being a useful aspect of the main plot when it touches it at a certain point. The characters are not as finely drawn as in the Vance novel but they are every bit as good as they need to be, and they are presented in a sufficiently vivid series of situations, that the reader can round them with his imagination.
This is a good general principle: sf provides the special situations, and the situations provide the opportunities for romantic drama. More so than in mainstream literature, the characters can afford to be stereotyped because the situations confer vividness on them to a special degree. Thus the so-called "stereotypes" become brilliant in their simplicity, like gemstones.
Examples that come to mind include:
The beautiful-but-mean-minded Sanoma Tora and the noble-hearted Tavia in E R Burroughs' A Fighting Man of Mars; the various women who beguile or infuriate the hero in Ward Moore's alternate-history novel Bring the Jubilee (in which the Confederacy won); the awkward situations in which the space-pilot hero finds himself in J T McIntosh's One In Three Hundred, where the Earth is about to be destroyed and space-pilots are given the task of choosing which few people are to be given a chance to escape to Mars.... a situation rather conducive to acquiring a lot of sudden girl friends.
In the Ooranye Project there is the dilemma explored in "The Open Secret", in which belief in reincarnation is an obstacle to a romance which, in a previous incarnation, led the hero and heroine to produce a child who grew up to be a destructive arch-villain who plagued civilization: the fear is that if they marry again in their current lives, their evil child will also be born again. And in "The Forgetters" the theme of love is likewise basic to the plot - the hero seeking artificial amnesia as an escape from an unhappy marriage, with unforeseen results.
However, sf can do more than all this. The theme of Love itself can be brought explicitly - if you like, philosophically - into the story. Here I must mention the only woman writer to be cited in this article - C L Moore, author of two of the most unforgettable sf tales ever.
"No Woman Born" (1944) is the story of Deirdre, a dancer and singer, who was all but killed in a theatre fire - only her brain survived, and she was given a new body of metal. Its designer knew better than to try to reproduce her old body scientifically; but science could give her another kind of beauty. The metal body is marvellously described. Its fluid, serpentine motions enthrall her audience when Deirdre gives her first comeback performance. Yet we are made to feel that Deirdre is doomed to drift ever further from humanity as the metal of her body influences her soul. The love in this story - the love, not of two individuals for each other, but of an adoring public for a diva - is bound, we feel, to end tragically.
And finally, the ultimate claim for romance, for the power of love: "The Bright Illusion" (1934). Due to a cosmic conflict and the subterfuges and disguises which it makes necessary, a man and an alien are brought together in a love which transcends the barrier of species. Their bodies are horrible to each other, but their souls somehow become enamoured.
It was a very strange sensation to be addressing her thus, from brain to brain. "The sight of you was dreadful to me, and I know how I must have looked to you. But the shock of that sight has taught me something. The shape you wear and the shape you seemed to wear before I saw you in reality are both illusions, both no more than garments which clothe that.... that living, vital entity which is yourself - the real you. And your body does not matter to me now, for I know that it is no more than a mirage."
In a sense, C L Moore is making opposite points, taking opposite sides, in these two great stories.