Ok, I played the demo, and here's my take on it (just my personal opinion though):
The big thing that strikes me as odd is the setting premise. Because it leads to many other problems further down the line. Succubi are evil and having sex with them just once will kill you. First, that means it's a black-and-white, good-vs-evil scheme. The antagonists have no ambivalence, no redeeming qualities. They're cliche villains, driven only by a desire to kill (through sex). That doesn't exactly give rise to the most compelling storytelling, because there's no meaningful interaction possible with this kind of enemy. Other than killing them before they kill you, of course. But I didn't come for wanton slaughter, I came to fap and ideally get some nice, entertaining story to go along with it.
Second, even if it's not said so directly, this premise also hints the prudish, and arguably incorrect, notion that sex is evil (because it kills you). Which isn't exactly a message you want to hear in a game such as this. Even if it's unintended, it could be interpreted that way.
Third, and most importantly for an adult game: You've kind of painted yourself into a corner. The MC has sex with a succubus just once, and now there's no other choice but to kill him. Game over. That's what the premise demands. There's no slow creeping corruption to possibly fight back against (or embrace). It's not like you just lose willpower or strength that you now might have to regain. Or seek out a cure against slowly turning into a succubus yourself. Or anything. There's no path forward. Only death and reload.
Meaning: If you pursue the content that you originally came to see and that made you download the game in the first place (aka lewd pics and hot succubi action), you'll effectively get punished by losing the game and having to try again. The only way to win and progress is to avoid the content that you want to see. This game is not unique in that regard, many others have the same problem (bad ends as the predominant way to access content). Nonetheless, I think it's wrong. If you immerse yourself, you're never going to see the porn you came for, because it would mean "your" death. Personally, I believe it should be the opposite: The player who actually engages with the game's story and premise and achieves progress should be rewarded. Not punished. Play to unlock the content, not avoid it.
To be clear: This doesn't mean the game is all bad, by no stretch. The images are good and the author put decent effort into actual world building (which far too few developers in this genre do, if you ask me). All that work doesn't necessarily have to be thrown out completely to "fix" the game (according to what is essentially just my personal taste, of course). But I'd modify the premise somewhat. Maybe the MC has some divine blessing that allows him (and only him) to safely have sex with succubi. Maybe it's a demonic taint that'll protect him short-term, but corrupt him in the long run, unless he finds a cure. Maybe succubi drain your soul instead of bodily fluids and slowly turn you into a demon yourself if you overdo it with the sex. Which, by the way, would also allow for lesbian scenes that are difficult to explain if succubi are just after the cum. And what psychopath doesn't like to see some hot girl-on-girl action?
Maybe succubi aren't all single-minded killing machines and can actually control how much they drain people, if they choose to do so. Perhaps they have other goals and motivations as well. It would open up more options for story progress as well. As they are now, humans have to eradicate all succubi, and they don't even have to feel bad about it. Because there's nothing left of their humanity. If some form of coexistence was possible, suddenly you have more than one path for the MC. He can kill them all, or he can try and find another solution. Defy your orders or obey them? Trust the "good" succubi and risk the fate of humanity or play it safe and commit genocide? There's a lot of potential conflict in that. And from conflict, great stories arise.