When you write erotica the mechanics of sex are interesting. The what can go where and when becomes as much your focus as the actual story. For instance, you wouldn't want to write that you left the male submissive tied to a tree wearing nothing but a pair of shoes and then write that his cock was hard, pushing his panties out in front of him. Things have to follow along in order, logical order. It gets more complicated in a sex scene where two bodies are closer and all the arms and legs get in the way.
It's hard to keep watching for where the fingers are. Luckily people tend to have two arms and all those fingers. It makes it so much easier to have that second hand wandering after you leave the first one at a crucial point. Still, there is the matter of reach to be considered. Two arms attached to the same body can't reach from head to toe if the mouth in engaged on a nipple. It just won't work.
Then there is the whole general positioning thing. If both are facing each other you have all those body parts in good reach. But, turn one away and one person has a limited reach. It would be impossible to stand turned away from someone and reach back and grab a nipple. Well, not impossible but not easy and likely a strategically placed mirror would be a big help.
Which brings me to another tricky part, toys and equipment. You can't write about ball gags if you've never (at least) seen one. If you've never seen a leather mask how can you write about it's use? If you don't know how a whip is used can you write about whipping someone in a safe, sane, consensual way?
The mechanics of sex can definitely backfire on you in erotica. That's why it's so good to stick with what you know. Of course, you can also have all the fun of getting to know more, trying new things and exploring your options.
