While admittedly I do not produce porn games, I'm pretty sure the process is more like a one time session where she will provide a bunch of sounds which they will then be able to remix any way they need to for future content.
It's not like we are dealing with a heavily scripted story line that requires constant line reading.
Since this is basically just a demo of the combat systems, without any story included we have no idea what they will want voiced. I wouldn't put it past eromancer to want some grand story included based on what I recall from Malaise. And yeah, if it's just the generic porn lines that's able to be done relatively quick and easy.
But, if you want anything more than just generic lines / moans you need to know what you want the VA to say before you hire them (ideally). So they may want lines specific to a sex scene, like "NO, I'm fucked by a mutant robot dog", which would complicate things with a VA for scenes they've not decided to make yet.
So if they want the VA to do any dialogue for story purposes or beyond generic sex lines, they'd either need the same VA for the entire time of development (really only practical long term if the developer is the VA), try to get multiple VAs with similar voices (AFAIK it can be hard to get that for non porn genres, so not likely for a porn game), or do it all at once when you have all the lines you want written already.
They could get a soundpack of standard moans / whatever as a placeholder, but that would make some sense. That would also allow them to just name the final VA files the same as the placeholder names and switch them out.