Does it exist?
A cursory search on Wiki lists the Gor books and a few older pulp novels, but that’s it. And from what I can tell what’s listed is really more in the fantasy realm of literature.
Of course it exists. Look up the Saarm Cycle on your favourite search engine.
Isaac Asimov wrote a short story poking fun at Playboy Science Fiction, which was more or less what you think it would be given the series title thereof.
In fact it’s conveniently online. “What is this thing called love?”
Whoosh?
Literotica dot com
Early Phillip Jose Farmer.
Philip Jose… drat, ninja’d by Czarcasm.
On a related subject, there are at least two alien love dolls. I hear the boxes look way better than the contents.
Cecelia Tan has edited a few anthologies of SF erotica.
No, it’s a typo. He’s talking about the Swarm Cycle. It’s a shared setting used by several online erotica authors.
There is a lot of it currently being written by ebook authors of the erotic romances that are gradually taking over and replacing the romance novel market. Go to Amazon and do a search for “erotica” and “science fiction” and you’ll find quite a few. I’ve reviewed a couple on my blog, many of the books are just not very good:
My blog is VERY NSFW so I’ll spoiler the links:
[spoiler]
Anjanette by Candace Smith. One of the flawed books.
Slave Harems of Xhagia by Teagan Rand. Probably the best written of the bunch, in terms of plotting and structure and characterization.
The Punishing of Jhendri by Susanna Valent. Not really SF, more like a fantasy medieval Amazon alternate world, but a lot more porny than most erotic romances, to which I say, “Huzzah!”[/spoiler]
Of course, many Amazon erotic romances have reviews other than mine, though many have no reviews at all, but I think it’s fair to say my reviews are a lot more thorough than most Amazon “reviews.”
35, according to the Internet Science Fiction Database.
The paranormal romance market is larger than the science fiction field, and may be larger than science fiction and fantasy combined. That’s too much to make generalities meaningful, but the contents vary from placid to steamy. And as EC says, erotica is huge is ebooks. Subcategories galore. Monster Erotica. Gryphon Erotica. Reluctant Dubcon Tentacle Erotica. Reluctant Gender Swap Erotica Romance. Click and be amazed.
The creme de la creme of sf erotica is the delectable Space Nymph by Justin Lasser. This is the only title he’s credited with and only one copy of this book is available online that I can find. Why is this book important? Let me quote from the first page:
Commander Obi Oregano of the Knights of Gazza. Does he strip? Oh, yeah. A Star Wars parody from 1978. Almost as funny as Bored of the Rings, though not quite as intentionally. You get it all. Leia and Jabba. No, not that one. Jabba is the name of the handsome young Wooky. Leia and Lute, Obi’s brother. Leia and Lute and Jabba. Obi and Trina. Obi and Leia and Lute and Trina and Jabba.
The Future is Now.
Image of the Beast being the example that immediately comes to mind.
The boxes are the contents and who cares how they look? 
The MIT Science Fiction Society (MITSFS) has long had a collection of SF erotica. (In my yearbook they used a photo of the collection to represent their club). It’s kept under lock and key, and I think they lie about its existence to most people. I don’t mean the stuff by Philip Jose Farmer or Cecilia Tan. THAT stuff is downright respectable, by comparison.
These are the books that are written under pseudonyms and never seem to show up in the SF sections of any bookstores. I’ve seen such stuff over the years.
of course, now that the internet is here, there’s access to plenty of fan-written stuff. That existed in previous years, too, but in the form of badly printed 'zines that you had to know the creators of, or pick up at cons, or whatever. Now all you need is a decent search engine.
Lots of weird fetish stuff on the internet, and “slash” fiction, but there’s simple SF sex, too.
Look up John Cleve, a pseudonym for Andrew J. Offutt.
Actually, it’s a floor wax and a dessert topping. The alien menace is called the Sa’arm, but are more commonly called “the Swarm”. At least half the stories aren’t terrible.
Well, Offutt and Dwight V. Swain, G. C. Edmondson, Robin Kincaid, Roland J. Green, Victor Koman, Jack C. Haldeman, II, Vol Haldeman, and Geo. W. Proctor.
Offutt did a half dozen or so in the 1970s, but around 1982 sent out a general call to writers to come in and do books in the series. The rest of the books were by those others. I don’t know who did which, though.
It depends on how broad a definition of SF you’re willing to accept. Most of Tan’s anthologies are fantasy or horror (or non-SF) rather than science fiction. I’d hate to see somebody led astray and getting elf porn or vampire porn when they wanted cyborg porn.
That reminded me of the Television Sex Story Archive (link to their SFW front page). They have a science fiction section and you can get x-rated fanfics based on most SF movies and TV series.