anything that leads to ntr due to mc's failure to overcome some kind of challenge (and eventual corruption of the fmc). the "seeing" it happen without having a way to reach\interrupt is a plus
All of the Kegani lab games
Timeloop is probably the best
https://f95zone.to/threads/time-loop-ntr-v1-40-kegani-laboratory.82733/
Welcome to the sexy bar is decent too
https://f95zone.to/threads/welcome-to-the-sexy-bar-v1-6-0-kegani-laboratory.58782/
Other games by them are ok but definitely lower tier
You also have leane 2, this one is great have multiple girls and lots of events with them before NTR happens (be warened it have a very long intro before any Ntr can happen though (10-20hrs)
https://f95zone.to/threads/leane-2-leane-of-legitimate-crown-v1-51-makura-cover-soft.49620/
Also got NTR adventures, really enjoyed this one too actually found it very difficult to avoid ntr on 1st play through, be warned mtl is very bad, so if possible re-translate using deepl yourself.
https://f95zone.to/threads/ntr-adventurers-lyne-elza-final-rain-shelter-frog.107413/
Also got some alicesoft titles with proper tl
Beat Blades haruka
https://f95zone.to/threads/beat-blades-haruka-v1-37-alicesoft.455/
Beat angel Escalayer
https://f95zone.to/threads/beat-angel-escalayer-r-final-alicesoft.54399/
https://f95zone.to/threads/spirit-s...ay-ill-really-end-you-final-bbq-lover.111524/
This one is pretty good although to avoid it choices are more binary (basically spot the guy to stop him or ntr)
https://f95zone.to/threads/okaken-no-hihou-kyoufu-no-ntr-sugoroku-v1-03-moccori-factory.79125/
Really enjoyed this one, unique gameplay, rng dependent, but also get items you can strategically use, quite short
https://f95zone.to/threads/shadowcore-shadow-garden.11356/
Another old classic, mtl is pretty bad but not the worst
There are also a few untranslated gems:
oyako rankan - one of the best avoidable NTR games
Make me lover - also really enjoyed this one too, however maybe look at a guide if you want to transfer from the pure love to ntr route as requires loosing specific battles (if not just get a game over). Or can just play the darkness route to start full ntr.
N love - this one was good too however felt a bit short (pretty hard to get ntr'd unless actively choosing bad options)
^recommend using textractor to play the untranslated ones
All of these are when the player has a option to avoid the ntr and it occurs due to bad gameplay
Probably missed quite a few more
If you want ones where the MC loses the girls but is scripted (no option) there are loads more