I have run across a few decent ones. For instance, I just played Episode 10 of Halfway House yesterday and it did a decent job during scenes - some heavy breathing and light moans, breathing that picked up pace, moans and squeals that got better and more frequent, until a nice finish. And it wasn't constant: some breathing, quiet, a little more breathing, a moan, a sharp gasp as a full thrust was made, quiet, more breathing, heavier, quicker, quiet, etc. until the final sequence where the breathing and some moaning became constant for a few seconds and built until the end. My only criticism is that the breathing had more of a male tone rather than female, and the females were the ones that were doing the heavy breathing. I think a couple of different voice actor clip sets were used for the moans, gasps, squeals - that or the Dev was able to modulate them a little so it sounded like at least two different female vocalization sets were used for the five different LIs that had scenes (never more than two per scene).
Can't remember which one it was, but another I did not too long ago had a similar pattern, but included some pelvis to ass slapping noises and the like, which were done pretty well.
It can be done well, it just so often isn't.
What I don't want are people actually speaking the script - the variety of voices usually isn't broad enough and they never match the voice I gave them. This happened in The Awakening, I think, where voice actresses were introduced in the newer updates and they generally didn't match my imagination, and the "younger" sounding voices used for the 40-ish character, as opposed to "older" sounding voice used for the 20-something just didn't sound right regardless of what I had imagined before. It may have been another AVN, but it was one with the older/younger dynamic and a mismatch between who read which character.