I also don't like real porn images in games. My guess for a reason they are used is to provide a visual stimulus for the reader rather than leaving them without a visual stimulus.
Ms.Denvers. It is a female protagonist game where I feel the choices actually make a difference. It kind of follow the "innocent girl turned into a slut" pattern, but that would be because of choices you make.
I've seen single option choices in VNs. Only a few times I seen it used to mark some significant event in the story. A lot of other times, I see it as a pointless choice.