I think either approach has merit. There are several sexual fetishes that I enjoy that either are not actually possible (cervix and nipple penetration, for example), or are a bad idea for health reasons (say, ass-to-anything or throatfucks that last for multiple minutes without breaks for breathing). One advantage of sex stories over real life sex or live action porn is that the things you write don't
have to be realistic. If you don't want to deal with STIs, or pregnancy, or the messier sides of anal sex, or the physical constraints of the human body, you can just pretend they don't exist. In fact, not only
can you write widly unrealistic things into a story, in many ways stories are the
best medium to depict them. The suspension of disbelief is pretty robust.
That said, just because you move beyond the physically possible doesn't mean the other guidelines of writing suddenly cease to exist. Your stories still need escalation, your characters still need to grow (no, not in that way!), your plot lines still need to progress. If chapter 1 starts at an 11/10 on the intensity scale, then when you want to raise the stakes in chapter 2, you'll have nowhere left to go. Even a story with as simple a premise as
Blowjob Universe eventually had to deal with the realization that, well, "guy gets to throatrape every woman he sees" needs at least
some evolution to remain interesting. (And please don't misunderstand, I don't mean 'simple' as a pejorative. Simple stories can be fantastic, and Blowjob Universe is a great example of an amazing yet simple story.)
For my own work, I've generally tended to the realistic, even when the setting was a fantasy world. However, that's less a conscious artistic choice than a deep-seated personal compulsion for things to Always Make Perfect Sense. I do have several half-formed plans for more fantastical sex stories, exactly because I do at some point want to overcome that limitation and explore just how much you can do when you throw realism out the window.