I recognize I probably put way too much thought into this. That out of the way ...
I think Veronica may use the Master/Slave relationship to cover her problems with the incestuous relationship with her brother. When MC and her are together, she's not with her brother, she's with her "master." Not much of a distinction, but real people have used way less to hide from situations they don't want to admit to. With the master/slave setup, then anything that they do is on the master, and she is blameless. To be fair, Veronica never really shows any problems with the relationship in the game, that I recall (though there's not a whole lot of useful conversation with her before the master/slave setup is introduced), so this is total supposition and trying to understand why she wants the master/slave setup, even when MC offers to drop it when they become full boyfriend/girlfriend. I like things to make sense, so this is me trying to understand something of her motivations. There's complete trust between the two, and their relationship seems the most developed in the game.
On the flip side, Amy is kind of weird in that she doesn't seem to think anything's at issue for MC to have a relationship with his sister. Doesn't say a word about it that I recall.
With Mom, obviously she has no sexual outlet, and has the excuse of the fake Dr. emails to fall back on to cover her actions with her son.
Ashley is just a hornball, as far as I can tell. XD
Again, probably putting way too much thought into this. *shrugs*
