I am doing some writing after a rather lengthy pause and discovered that I feel... bored when writing sex. The mechanical part - poses, A going into B, orgasm descriptions, tempo and angles. It does not feel like a creative, exciting activity but like a monotonous task. It feels like I am adding nothing new and merely rehashing and rewriting scenes I have already written in one way or another. I still like writing erotica. I like writing a story. I like building up to sex. I like the aftermath scenes. But when it reaches the point of writing a sex scene, it feels like "here we go again" not like something I enjoy doing. Have you experienced this? Have you fixed it in some way? Perhaps it was just a phase that went away?
I'm still at the stage where I don't even feel confident writing sex scenes in the first place, but I can already see how it could be difficult to keep coming up with fresh ways to put them together. And at a certain point, if you try too hard to write something new and original, you could easily go too far and just make something weird that takes the reader out of the action. Is part of the trick that, even in an erotic story, not every sex scene has to be written in detail? I can certainly imagine situations where I'd leave the physical parts mostly up to the reader's imagination and focus on (as you say) the buildup, the scenario, the relationship, how the characters feel about what happened. The detailed stuff would feel a bit more special if used sparingly sometimes.
Sex isn't doing it for me, so most of my stories have very little actual sex in them. I don't think you need sex per se, as long as you make up for that with plenty of otherwise erotic scenes. This of course depends on the type of story you want to write. Something in ENF works perfectly fine without a single sex scene in it.
I've had the same issue in my writing, and my preliminary conclusion is that it can get boring to write because at its most fundamental level sex is a mechanical, monotonous task. There's only so many ways you can eat someone out or give a blowjob. To get over it, I've tried to do 2 things. One, write less of it, as lady-lux also alluded to. At one point, I was writing sex scenes that topped out at 10,000 words or more. At a stiff (ha) 500 words per hour, that was 20 hours for 1 sex scene. That's ridiculous! I really didn't need to (and frankly shouldn't have!) dedicate 2 paragraphs of prose to "she inserted a finger", 2 more to "she added another finger", and then another 2 for the third finger. No wonder I got bored of it. I like quantifying goals as concretely as possible, so my current target is at least one orgasm every 1000 words (and preferably half that). Two, focus less on the mechanics and more on the emotions. The thing about sex is that it feels good, and even if you do the same thing this week as you did last week and the week before, it feels just as good... on a physical level. But cocks and pussies are not our main sexual organs; our main sexual organ is the brain. Engage that and you'll have a sheer endless variety of sexual nuance to choose from. Have your characters talk to each other. Have them make jokes. Consider how they would turn each other on. Dive into their psyches. The piv is not the interesting part of sex. It's everything else.
