I don't particularly care about the MC's dad fucking his mom. It's his dad...why would they not fuck? Maybe I'd have a problem with them fucking if I knew that he was either a complete asshole to her, if he had a lover and he just fucked Amy just as an "obligation", or if he used violence against her. That was nice and all, but it didn't seem to add anything to the story. That's about the only "other guy" I'd tolerate having sex with one of the female NPC's(specifically his mom).
I feel that the POV question was unnecessary because I've never identified myself with more than one character, unless it involved a game with a serious an incredible story (not a porn game). For obvious reasons...in porn games/adult visual novels(or whatever you want to call them), I only identify myself as a male MC, so I was just an expectator in Amy's sex scene, which seemed to be present only due to fan service. The only logical explanation to show that sex scene would have been if the MC was actually peeping on them having sex.
An adult visual novel with multiple POV, sounds like a real bad idea, because chances are that the story's going to spread thinner and thinner, and unless you were a superb storywriter, I highly doubt that you'd be able to create a compelling and entertaining story which actually needed to be told by mutiple POV, which doesn't seem to be the case. (for an adult visual novel)