Kicking this off with actual hentai and Inyouchuu related discovery from a few years ago, this music library contains several tracks from the Inyouchuu Shoku OVA:
https://www.youtube.com/playlist?list=PL4x9UMJPVN3zxWSHyPs4M7fNgUo3fe_hR

Then we found some Taimanin Asagi tracks as well, love how this nasty one is called "Insect" hah.

So aside from that, chalk it up to having too much perv time on my hand, liking dark ambience in general, and a quest to find hentai-like dark tunes that fit "the mood" and or sound like TrioDesign's dark H tracks. Stuff you could almost hear in the grinding of a tentacle orgy, or what a bio cave might sound like... here are some of my favorites.

Nine Inch Nails - Ghosts I - 3
https://www.youtube.com/watch?v=DApuMNoN3ME

Nine Inch Nails - Ghosts II - 15...  now we're getting wet 
https://www.youtube.com/watch?v=EJ51kW684FM

Michal Wolski - Ramp [IPF002] ... even wetter?
https://www.youtube.com/watch?v=5sk18pAOoKQ

Aphex Twin - Weathered Stone
https://www.youtube.com/watch?v=wvCLjFpe1Wo

Ai Limit - Scurry of Necros... something about this one sounds like the FINAL big bug birth is about to happen
https://www.youtube.com/watch?v=Y7pCuSdQ55I

Loefah
System: https://www.youtube.com/watch?v=Fo6V8rvmQ7g
28 Grams: https://www.youtube.com/watch?v=ojSNiGGORKI

If you don't mind some "deathstep" adjacent stuff, I love these:

https://www.youtube.com/watch?v=aHHh0iZpxi4
https://www.youtube.com/watch?v=5IgssHv8iv4
https://www.youtube.com/watch?v=nQOAAt4rNOs

I need to try making my own sometime. I could easily see dark cavelike ambience through the synth program I've been working on.

Also meant to say I'd love to talk more about the dark TrioDesign tracks sometime.

The one rare time I wouldn't mind tinkering with some AI, if I could plug some of those tunes in and remix them around for other similar new tracks for my own use, that'd be fun haha.

I love how the Shoku bonus episode just used music from the game and got all the good ones in there. Still think that's one of the best OST's. TrioDesign is amazing and I'll listen to some of the non-Inyouchuu games as well that have similar dark tracks for the mood. Sandoku had some good ones, and the Sakura's.

Is any of this available on Spotify? My music backlog is horrible. But I do love having options.

Not sure, but I posted some artists. I bet Nine Inch Nails is, but then finding these real specific songs might be a task haha.

https://www.youtube.com/@Hypnusrecords

Digging this channel lately, for more of that subdued low fi darkness you could maybe hear in some hentai scenes.

New cool one today I discovered... Bio Cave approved.