A bit of context: as far as writing porn for my games goes, the process is typically split into multiple stages over the course of several months. It usually looks something like this:

• First, I map out the overarching plot beats of the section of the narrative I'm working on (this went chapter-by-chapter on Opportunity, and will work a bit differently on my next game). Since my personal brand of porn trends towards the naturalistic, I keep an eye out for places where a sex scene could convincingly occur - I might have specific things I want to occur in a given chapter (such as "I want to find a place for a MFF threesome") but usually I like to let the natural flow of the story dictate what kind of sex happens (the sex feeling like it "makes sense" will also make it easier to write later). These are VERY high-level plot beats, so at this point the sex scenes are literally "and then Bob and Jane fuck down".
• Once the plot is sketched out, I get to work expanding on the sex scenes in detail. Because this is a visual novel, I need to think of the beats of a sex scene in terms of the artwork that will accompany it - there are a lot of different terms for this kind of art in VNs, with the inside-baseball term being "CGs" which stands for "computer graphics" and which I have always thought is a very stupid term for it. Generally I think of the art as either "splash art" or "panels", but the BEST comparison is probably "keyframes". Anyways, at this point I'm just going through and mapping out the major visual events of a sex scene for the purposes of Pacha eventually illustrating them, but I'm still not getting super detailed, so we're talking stuff like "Panel 1: We're looking at Bob's crotch. Bob is wearing pants. Panel 2: Bob has dropped his pants down to knee-level and we can see his erection through his boxer-briefs. Panel 3: Bob has got his nasty thang out of his boxer-briefs (remember: precum)". This is a step of the process where it really pays to be horny and self-censor as little as possible, because I'm going to have to think about these fucking things a LOT over the next several months so I might as well have fun with them while I can (it's also very important because it gives me an idea of how much art time is going to be necessary for a given milestone)
• After the sex scene panels have all gotten this kind of rough description, it's now time for me to create & gather reference material and beef up the descriptions. I use the Magic Poser web app to create 3D reference images for whenever there's a significant change in position (so for example, I'll create a 3D pose reference for panel 1, but if panels 2-5 are just small variations on panel 1 then I won't bother unless there's a very specific movement or something I want to highlight), and will also find detailed reference images for clothing, furniture, backgrounds, and so on. The written descriptions will go from "Panel 1: We're looking at Bob's crotch. Bob is wearing pants." to "Panel 1: We're looking at Bob's crotch from a 3/4 angle. Bob is wearing brown corduroy pants. This scene is taking place in the cathedral, on the front row of pews. See "scene1Ref1", "brownpants", and "bg_cathedral" in reference folder." Once everything has gotten a beefed-up description and a full suite of reference material, I pass it off to Pacha.
• At some point, Pacha gets back to me with sketches of the scene, and I either okay her to bring them to full rendered color or make tweaks. This is also a good time to add panels if I think they're necessary to make the scene flow better.
• Pacha then gets back to me with the final color versions of the panels for me to either nitpick or accept. Typically I wait until after receiving the final panels before actually writing the full version of the scene, since a lot of what I write is going to be informed by the visuals and I don't want to have to do substantial rewrites because the panels look different than what I was imagining in my head.

So, to get to your actual question - the longform, long-TERM collaborative nature of the kind of writing I do means that by the time I'm actually doing the work of writing the full-length scripts for my sex scenes wherein I provide gushing descriptions of wet pussies and glistening asses, I will have been rotating the events of those scenes in my brain for something like 3-6 months. While I'm not going to pretend that I've never gotten horny while doing this writing, the novelty has definitely worn off and at that point it's really more about the craft, delivering on the potential of the rough description I created in the early stages. It is typically a great relief to actually get to WRITE the stupid things after all of the planning and waiting.

The actually-actionable piece of advice I have for you at the end of all this is to at very least outline your sex scenes in a similar fashion as to what I described above. You don't need to get super granular with it, but sketching out your intended vibe ("this is a rushed, frantic hookup", "this is an ill-advised, long-running affair between secretary and boss", etc) as well as the big beats you want to hit ("staying late at office, door and blinds closed, dark, start against wall, tear shirt open, biting neck" etc etc etc). This will give you a recipe of sorts to follow, will hopefully solve the problem of you losing focus while writing (since the events will at least be partially anticipated), and will hopefully help you translate what's in your head to something other people can parse and appreciate (and also jonk out a big cumb to)