Everybody likes to look down on someone. There's an inner elitist in everyone, but many people also choose not to project their judgements onto others.
Absolutely right... Meaning the distinctions between genre fiction and mainstream fiction hardly matter when it comes to the elitist posing. Within mainstream fiction, you'll have someone look down their nose at, oh, say, an author of thrillers as opposed to someone who writes stuff for the New Yorker. Readers of the romantic novels on the mainstream fiction shelves might likewise dismiss those on the shelves labeled Romance, although what little I've read in my wife's bathroom collection (Nora Roberts is the only name springing to mind) reads at the same pace and level of crisp pose as most any thriller does.
We could weigh in the higher calling of contributing something memorable to the world's literature, but not without getting into the kind of b.s. territory the original post did not address. The genre of one's fiction does not dictate this kind of quality.
The mainstream fiction shelves at B&N puzzle me. All of Vonnegut's work, the bulk of which is soft sci-fi, is in the general fiction section but not F/SF. There are a few other examples (Christopher Buckley's sf satires are all that comes to mind just now). And of course all the thrillers and horror novels are absorbed into the mainstream fiction aisles, which is funny too. Many of them, especially the likes of Stephen King, cross over into F/SF more often than they don't. Fine as all these authors are, I can only chalk up the category mixups to artificial prestige. I find myself wishing that these works were "ghettoized" into separate sf/f and/or horror/thriller aisles at least for my own convenience.
That having been said, knowing how much (if not most) of the world's greatest literature throughout history has incorporated myth, fantasy and science fiction in various mixes in the attempt to better capture the human experience through words, we all know that someone who disparages SF/F based on genre alone is hardly the literate person they're posing to be. I only
wish it didn't matter, as some would suggest. Trouble is, as long as there is any commercial or marketing difference, moronic impressions of genre fiction can negatively impact writers of this material.