The Erotic and the Speculativebyleapingfox©
SF (speculative fiction; the category includes science fiction, fantasy, and related genres) has a lot in common with erotica. Both are ridiculously easy to write badly and excruciatingly difficult to write well. Both are widely considered "inferior" by the mainstream literary community, yet actually contain some of the finest literature in existence. (Vonnegut's Slaughterhouse Five is about space aliens and time travel, and Joyce's Ulysses has such explicit parts that it was originally published as porn and banned by the US Supreme Court. It is a sad truth that both books are usually considered somehow "transcendent" of their roots, not "just" SF and erotica but something "more" than these, as if SF and erotica aren't good enough to be counted as literature.)
The subgenre of erotic SF suffers from the challenges of both its parents; the combination of the two makes it an extremely difficult genre to succeed in.
Yet with this challenge comes reward; once you lay down the taboos of your culture (erotica) and the facts of your human experience (SF), you suddenly become free to write about almost anyone, anything, anywhere---of course including people and places and things (and things that are part place, part person, part thing; think Moya of Farscape or Andromeda of, well, Andromeda) that are not, never have been, and cannot ever be. This incredible freedom quite literally can take you to the stars, back through time a thousand years, and infinitely beyond.
So in that sense, yes, you do abandon the need for "realism" to write erotic SF; but do not be so quick to think that it is entirely gone. Realism leaves a shadow on SF that remains important.
The most important feature of this new realism is consistency. If the world you create---for it is worlds you are creating, whole worlds that have never before existed, and you must create them well---if that world contradicts itself, your readers will simply be unable to engage with it. If the same alien technology or magical incantation behaves differently the second time than it did the first time, there better be a reason; or your writing simply won't make sense, and no one will want to read it.
Another crucial part of this realism is imperfection. Everything is imperfect. Everything. Always. You must write imperfection into everything you do. Even if it is possible for a world to exist that contains something truly perfect, a human reader, who knows her own imperfection all too well, will be incapable of interfacing with that world. Its perfection will be opaque to her, and ultimately, uninteresting. It is infinitely better to have characters with flaws, who make mistakes, judge wrongly, even behave cruelly, than to try to imagine some pristine utopia. The world you make can be very different than ours, and on balance be better or worse; but unless it too has both its good and its bad, your reader will be unable to identify with anything or anyone within it.
Then, there is of course the matter of anatomy, as is often necessary in these how-to manuals for erotic fiction. In erotic SF, you are obviously no longer limited by the constraints of the human form. You may change the dimensions, multiply the organs, even invent wholly new structures the likes of which have never been seen on Earth. Hermaphrodism is almost bland by comparison with the options available to you. I have envisioned races with three, five, or seven sexes, even beings that reproduce by asexual cellular fission yet engage in acts of physical intimacy purely by choice. And of course there is always the option of taking someone else's creation---an elf, a fairy, an alien from Alien---and transmuting it by the power of the pen into an object of lust.
Yet whatever you create, you must create it consistently, following your own rules---if it is normal for the R'Neeki to have three four-meter penises (do you realize how huge that is? You could go up and down my leg twice!), then it is normal for R'Neeki to have three four-meter penises. A R'Neeki female would not be particularly impressed if she encountered such in her lover. It would be like seeing a man with a six inch erection; certainly fine, especially as part of a whole package, but nothing particularly remarkable. Surely a human woman's reaction to our average R'Neeki male would be quite different---my guess is that it would be one of terror, but that should depend upon how you want to characterize her; maybe she is a xenophile, aching for the chance to have a trio of alien cocks wrapped around her body.
Another challenge unique to SF is the danger of spending too much time building a world or a language, too many pages spent on describing every facet of the environment and inventing new terminology to suit it. This sort of world-building is often far more entertaining for the author than the reader, and for that reason should be largely relegated to your private notes (which ye shall have, always) and not published in the final product. For the depth of understanding it gives you, is certainly good to know the muzzle velocity of a Solar Federation Marine's bolter machinegun, but you don't need to actually state it in the text of the story, and indeed if you do, you'll probably stall the flow of the narrative. (That is, unless the muzzle velocity of that weapon somehow becomes crucially important to the story... I can't imagine how, but part of the fun of SF is trying to come up with things that no one else can imagine.)
One thing you must never think is that depth of characterization is somehow optional. Depth of characterization remains every bit as essential in erotic SF as in any form of literature---all the more so for the fact that it can easily be overshadowed by the alien worlds and exotic sex scenes. This is, in large part, what separates the authors from the people who think they are authors; only the former realize that despite the glitz of interstellar travel and the heat of tentacle sex, you still have to write a good story about interesting people if you want people to read what you've written.
The rest, I leave to you; I wish you luck on your journey.