I'm sorry to hear that you found the game lacking. =( If you have any feedback on how we can improve in the future, please let us know.
First of all, I would tell more of the story (if it is important to the context) through the combat dialogue. Instead of having a bunch of repeat dialog the player wishes to skip as it gets repetitive, have a purpose to the character's banter. I found myself starting out reading the beginning section, and then just skipping as I wanted to get onto something more interesting. As for the plot, try not to tell a porno story. Losing is a trap, losing later on is a trap, winning was conquest. A situation where the person playing cannot lose has no purpose. You're feeding into a certain crowd I know, the ones who want harems with every girl lusting after dick with no effort. Well, if you want people to take your story seriously; then one of the ways to do so is give failure meaning. Have the demon lose to her weakest state and have her dom him would be an easy one. If your crowd does not desire that outcome, they will work and develop a desire to beat this adversary. They will be stimulated simply by having perverse imagery in front of them more often than not. But they'll now be mentally stimulated by this drive for competition, for conquest, in a more driving manner than "I want to see the sex scene." It will be more of, "I want to see this bitch humiliated." "I want to see my enemies driven before me and hear the lamentations of their women." is a quote from Conan The Barbarian, that a lot of people love. Why? because everyone has had an adversary that they hated and anyone can envision all those who serve as obstacles to their rise to power crushed.
As for gameplay mechanics, first of all I would provide tool tips for what each action does specifically. If you want to not have to provide a walkthrough for the game, try giving the player the info he needs without spoon feeding it. Hell, you can even leave hints in the dialog and banter that goes on between the demon and the knight. The repetitive dialog really kills the immersion of the fight, but I talked about fixing that earlier so I'll just note that here.
As for sex scenes, variety is the spice of life and kinks are easy for variety. As stated earlier, femdom, could have taken place if the player lost to the earliest stage. If you truly want to have a scene where you trap the knight by letting her beat you, then have that be an option even labeled, "Let her beat you." As something only available in Stage 1 so that it's launched by selecting said option. Otherwise you leave the player not really striving for success, when failure is rewarded with success. The animations are fairly lacking, no so much in art style but in a lack of committing to a concept. If you have a demon and want to have a demon, show the demon. If you want the demon not to be too unattractive, give him more human features with minor features like horns and sharp teeth and large red body as you seemed to go for. The floating dick is almost insulting to the audience because it appeals unequivocally accepting to their tastes and shows no respect for them. As you are too afraid to show your story you wish to tell and show what the fight builds up to. The showing of the body would also allow for more in-depth scenes than, in-out-in-out cum-and-done. You can have gripping, slapping, spanking, groping; instead since you have only a floating dick, all we have is different positions to which the knight seems to be only slightly perturbed to be in (when not mind-controlled mind you.)
Anyways, I hope this info helps you. I gave it some effort and could go into further detail but I need to get to bed.
