It seems kinda pointless to make a VN without an actual story, because at that point you more or less have a fancy image viewer or a bunch of disconnected typefucking. If you start stacking real game mechanics on top of that, then you don't really have a VN any longer, but if a person's still calling it a VN then it's the N part that should be getting the attention. That's my opinion on VNs, being a guy who doesn't ever read them.
When you're talking about actual games, sometimes it actually feels annoying if the porn is too deeply integrated into the story. My reasoning is that it's pretty easy for things to start feeling contrived if every instance of porn is somehow tied into the narrative. I don't have really any specific examples, but I guess my feeling is that if I (of all people) am rolling my eyes at the plot, then I'm probably not enjoying myself unless I can ignore the plot entirely. I dunno. I have a hard time explaining this coherently without just saying that I prefer story-free porn. Sometimes or even a lot of the time that's the case--I don't necessarily need some overarching epic narrative for a porn game to be entertaining, but having interesting and consistently-written characters helps. I don't necessarily need some unified story to go with them.
My other problem is that integrating porn into the story kinda puts limits on emergent porn gameplay. Linearity of story is kind of a different issue entirely, but with a linear plot and heavily-integrated porn, you're likely to trip along from event to event, and then the game and the porn's over. I guess I'm getting used to more procedural porn (kinda like text-porn interactions, even though I think you wouldn't technically call it procedurally generated) that exist sorta outside of any larger narrative. I've been playing mostly slice-of-life/trainers lately, so I guess that's where that feeling is coming from: beating a bunch of personality-free ciphers against game mechanics until porn comes out of them.
I dunno. I feel like it'd be easy to say that I'm thinking of the problem backwards. I mean, it's easy to trivially counter my complaints by saying that a good writer wouldn't fall into the kind of traps I'm talking about, so what I'm really complaining about is bad writing. I guess I more or less am doing that, and if the porn's integrated too closely into the plot then a badly written story is going to drag the porn down along with it.
I'm trying not to sound too contradictory. I play porn games for porn more than the story, or honestly even gameplay. That is, I personally make my porn game choices based on the porn first, gameplay second, and story last. For instance, I don't read VNs, so throwing some porn into one isn't likely to get me to read it. But at the same time it's pretty clear that, for instance, both excuse-plot porn RPGs and epic RPGs with some porny moments have their audiences, so I can't say everyone out there feels the same way as I do. I guess that's why it's nice to take a poll about it.
Anyhow, that's enough out of me.