Here's a writing exercise:
Write a story in which a male human meets a female human, and neither one falls in lust or in love with the other, and they don't become a couple or have sex. (Don't make this the point of the plot, just a part of the story.) Oh, and neither one is a vampire or other hunter trying to seduce and kill someone, and neither one is evil.
For bonus points, make them both straight or bi, and make them both reasonably attractive (at least to other people), so that it's not that they find each other a major turn-off, just that they don't happen to hook up. (The thought can cross their minds if appropriate, but don't have anything actually come of it.) For even more bonus points, have them become friends.
My underlying thought here, in case it's not clear, is that in an awful lot of stories with a male viewpoint character, the minute a woman appears (usually in the first scene or two), she's obviously meant to be the love interest (often signaled by blazoning or other spotlighting of her appearance). And vice versa with female viewpoint characters. This isn't wrong or evil; it's a very common trope, and in some genres (like some subgenres of romance) it's pretty much a requirement. And it's nicely compact if you're writing a short story. But still, I always find it refreshing on the relatively rare occasions when the first prominent male character in the story and the first prominent female character don't end up as a couple.
Partly I'm reacting to the obviousness and predictability of it, as a reader; I like the idea of sometimes not relying on standard tropes.
And partly I'm reacting on realism grounds; men and women meet in real life all the time without ending up in bed together, sometimes without even being interested in each other (despite being of a gender the other can be attracted to). When two non-gay characters of the same gender meet in fiction, all sorts of things can happen. The two become friends, or colleagues, or enemies; they give each other useful or useless information; they interact in all the ways that humans interact. So I want to see more male and female characters interact in all the ways that they interact in the real world.
Yeah, yeah, insert discussion of When Harry Met Sally here, there's lots of room for playing with these tropes. And I imagine for some of you, this exercise would be trivially easy and/or the last fifteen stories you wrote would qualify. I'm just saying if your impulse is to always have one male lead and one female lead and make the two of them a couple, try stretching the boundaries of that, as an exercise.
Of course, in some sf stories, the flip side of this comes up: there are male and female characters, but there's no hint of sex or romance in the story at all, which generally means all the characters are treated as if they're male and entirely asexual. Usually they're all hyper-rational types whose role in the story is to solve a problem rather than to behave like a real person. But that's a different kind of story than what I'm talking about in this entry.
There's a side/sub-topic that's also worth mentioning here: I occasionally see a story with a male protagonist in which another male character appears and is obviously meant to be the love interest. (Or at least the protagonist scopes him out.) (This also sometimes happens with two female characters.) This can be a nicely compact way to let the reader know early in the story that the protagonist is gay, and I'm all for that. On the other hand, it also conforms to the standard romantic-interest storyline, so I roll my eyes a little at this too. Still, I don't see it nearly as often as the male/female romance version, so for the time being I see it as a nicely unusual variation.
(I wrote most of this way back in September, but never got around to posting it; I figure post-Valentine's Day is a good time for it.)