Well, I personally don't like anything that is too dark, misogynistic or sadistic, no matter how implausible it is. The real question you should be asking is why do people even need stupid plot devices such as blackmail, hypnosis, drugs etc. in a porn game. Why can't the protagonist be someone with a slutty, promiscuous personality, who just loves having sex? Why does she have to go through some horrifying experiences to become that? The answer to this is question is what really concerns me.
I also don't like anything that is
too dark, etc, but that's a mere truism. If I liked it, then by definition it wouldn't be
too dark.
Games with "with a slutty, promiscuous personality, who just loves having sex" exist in abundance. Play
Tamara's Exposure, for instance. The character there gets naked in public, gets caught, has sex with the person who caught her, goes home, and does it again next day. It seems to be reasonably popular, but I was bored by episode three.
I agree with Tennessee Williams that "the only thing worth writing about is the human heart in conflict with itself." I like characters who face some sort of adversity and war within themselves as to how to respond. In FMC games, that often comes in the form of "how far am I willing to give up self-respect to accomplish my goal" or "can I stand myself if I acknowledge that I like it," or similar themes. These are always internal dialogues, which are the best dialogues if they are done right.
A game needs a trigger to get the MC to a point where they can't stay in their comfortable original world, but must change. Often, this will be adversity in the form of blackmail, hypnosis, etc, because they are easily set up and easily understood, even if cliched. Porn novels use the same mechanisms, by and large, but can also take the time to set up a scenario involving sudden poverty which games seldom use because it takes so much buildup.
Long story short, blackmail and the like can quickly set up the internal conflict that alone (IMO) makes any writing worth reading, games or not.