I've got a few things here, but I think they've all been mentioned already Doesn't hurt to add my opinions though, I guess.
1. Having to loose to see the sex scenes. Why are you punishing me for wanting to see what is essentially the whole point of playing this game?!
2. The overabundance of rape/abuse. Yes, I get some people like it and I have no problem with that (in games - real life is whole different matter) but I generally prefer more consensual stuff. I have a hard time finding any decent games that don't use rape as one of the primary ways of getting sex scenes. I'll tolerate it to some extent, but when it's mostly all rape, I get tired of it really quickly. I don't care if it's lovey-dovey or "Hey, you! Want to screw?" "Sure!" This is the reason I kept backing Breeding Season right to the end, despite being unhappy with the development - everyone is enjoying the sex.
3. Surprise fetishes or pairings that I didn't know were coming and I couldn't avoid. I'm a straight guy. Give me M/F, F/F, Futa/F, M/Futa, etc. But M/M, or Futa/M (or strapons, etc), especially if it's supposed to be a self-insert character, turns me off really fast. I have no problem with it being there, but if it's the result of a choice I can make, don't surprise me with it. Same goes for hard bondage. If it's unavoidable, I think having a list of unavoidable pairings/fetishes separate from optional ones should be an important part of your description of the game.
4.
Isn't it also with most porn games that the player character is incredibly boring in and of themselves? I mean, I get it, they're meant to be self-inserts (at least to the extent that they're the player's avatar, to say nothing about actual customization for them) but as uncomfortable as I am playing male characters in general, I'd feel better about them if such a game provided me with an interesting backstory or at least a likable personality that made me root for the character and want to see them succeed on their own merits. I think that's pretty important, considering the vast majority of porn games is written from a male perspective for a largely male audience.
Let's play a little thought game on this: Assuming, for a moment, a regular mainstream game you like had explicit sexual content in it that you could pursue at more or less your own leisure with a character you like - let's say, Bayonetta or Dante from Devil May Cry for instance. Wouldn't you care more for them and their partners if you liked them as opposed to either being a blank slate? That's not to say blank slates are generally bad - they can work pretty well, but as with a lot of things in most adult games, execution is lacking.
This is possibly the biggest one for me. I don't mind having the occasional game where the player character is essentially just me, that can be enjoyable from time to time. But I really do prefer having them be an actual character most of the time. That doesn't have to mean that you don't have some say in who they are - Mass Effect, SWTOR and Dragon Age (specifically 2 and Inquisition) are good examples of letting you determine their character to some extent without bland self-inserts. (Yes, those are all Bioware - I have my issues with them, but I do think that player-character design is one thing they got right.)
Unfortunately, most porn games I've tried that do give the player character a personality seem to be one of a very small number of rather boring and (to me at least) unappealing options. Male: Complete ass-hole out to screw everything with tits, preferably by being underhanded/abusive. Female: Whore with no personality outside of "I like sex". Or either gender: complete pushover with very little personality at all, who gets raped by everything in existence. I end up not playing most games very long because I just can't get into an "rpg" that doesn't have a reasonably interesting main character.