As long as the difficulty is not artificial, then yes, I do believe every game should be allowed to have any amount of difficulty.
I do believe most people see the Porn Genre in games as a consumable, play once and forget about it after you get your fix. I disagree. I believe that before they are Porn Games, they will
always be games first, and that they must make use of the medium efficiently, if the sole difference between a porn game and a hentai on some website is what I click to progress (kinetic novels are iffy, but I'll give them the pass atm), then the game really has no point. I have no use for downloading a glorified slideshow when it could've been just as easily hosted online. So any games that make use of the interactivity, those must be looked at with respect and care, which is why I need to raise the point of Artificial Difficulty.
Artificial Difficulty is, to put it simply, working a 9-5 to unlock like 10 new important lines of text and maybe a scene. For example, any trainer game where you don't have a time limit and just have to repeat and repeat and repeat the same action so you can progress with a minor side event in a character's story is simply inexcusable, it belittles the player by forcing you to spend your valuable time doing busy work for little more than nothing, of course, doing this occasionally would be ok, but when all of your gameplay loop is just that, then you already failed as a game and you better hope your story is good enough to maintain the attention of the player, to me, porn is a part of art, and games a canvas you get to touch and feel, to glare at with love and heal the mind with, a game that challenges you with something unique and interesting is posing a challenge to you, saying that you need to earn what you want to do next, and that's what makes what you see feel all that much better, a grind is not difficult, it is boring and annoying.
If your sole goal when playing a game is a quick wank, I can understand how difficulty can be annoying, and that's a fine criticism to have, but to carpet difficulty as a whole as something bad would really devalue the value of so many projects with effort behind it,
Princess Reconquista is a wonderful metroidvania style game where some rooms and enemies are annoying sometimes, but the game as a whole is a game first and the porn is integrated so well with it as your way to heal by fucking the enemies into submission, it doesn't feel like something that was added later but part of the foundation all this time, it is to me, the gold standard of what a porn game should aim to be, interesting, fun and beautiful. Not all games have to be as difficult as it, but they should aim to make the game itself as tied to the art as it can be, an easy game can still be just as good as that one as long as it puts in the care to make the difficulty it has be something that isn't there to waste your time.
TL;DR; Normal difficulty is fine, grinding is not difficulty.