Skipped through the game and it's pretty much like MuchSalty1 said, only one of the girls gets corrupted.
Here's a quick walkthrough for the choices since it's pretty confusing:
Choice 1: Top ends the scene early and skips Choice 2, Bottom continues it until Choice 2.
Choice 2: Some changes in the CGs, but mostly unimportant.
Choice 3: This has multiple options to choose which type of CG you want to see first, however the second option always ends the scene so pick that one for last.
Choice 4: Again, changes the CGs but leads to the same choice afterwards.
Choice 5: This is kind of like choices 2 and 4 in that in changes the CGs, but you actually get an early ending if you pick the top option, so pick the bottom one to keep going. If you pick bottom this is where the corruption starts.
Choice 6: Kinda like 5, pick the top option to get an early ending, pick the bottom for the "true" end.
Choice 7: Post-credits, only changes the order of the scenes, still arrive at the same end scene.
On a foot note, Choice 5 has a variation of the end scene seen at Choice 7.
Overall the game is linear but puts these choices to trick you into thinking it's not, you can literally pick bottom everytime and you'll get to the ending with no problem.
