There are two questions to consider here(and neither of them are particular to h-games).
1) Are you depriving the developers of their income? If you would never have bought the game ANYWAY(for instance, if you just don't have much money to spend on games/porn) or if you pirate the game to try it out(much harder to justify if there is a decent demo) and then buy it later, then it's pretty clear cut that you're not harming anyone. This obviously gets a lot messier if you share the game.
2)Do the developers DESERVE that income? For hentai games it's generally a much flimsier argument(some devs may be dicks, but just because you're a dick doesn't mean you don't deserve to get paid - at least, as a general rule, I'm sure people can think of exceptions). On the other hand, I think we can all agree that the world would be a better place if Disney headquarters were burned to the ground and all their IP was made public domain.
1) If we put Patreon aside as a money-making concept, I think if the game is worth the purchase, then you definitely should invest in the developer via buying the game. The issue stems from some developers only packing maybe a few hours' worth of content beyond what you experience in the demo. While one could argue that finishing the game that fast is on the person paying, I have to ask why the game was so short to begin with. Also, VN's are the laziest form of "games" and I will fight you on that.
2) This point kind of ties into the first. As I pointed out, what's the depth of content? Is it just a few hours of content, or enough to last for a time? Will I replay it? Sure, a demo can give me an idea of the latter, but not the former. I have to know before I buy something if it's worth the money. I think we have to apply the rule of "separating the art from the artist" for the latter part of your argument, too. Jontron made some really bad statements about statistics a few years back. Pewdiepie has been incidentally associated with anti-Semitic/Nazi imagery. Both are still popular more as an indicator of their value as content creators and not as people. I'm sure some of Jon's fans would feel inclined to tear his asshole open verbally while singing his comedic "genius" in the same breath.
I think we can't exclude crowdfunding in this situation, though. Unfortunately, it has created a kind of sub-market of people who have "concepts" and may/may not have made Patreon their primary income. In that way, we have to ask the question of "do developers deserve to earn money?" Obviously, people who put their blood, sweat, and tears into something deserve some form of compensation for their hard work. How much do they deserve, though? If I'm putting together a game, do I deserve money without proving my commitment or competence with a previous project? Should my word be sufficient for you to hand over your money?
I think the complexities of piracy are murkier than "does someone deserve to get paid?" We need to examine the reasons of why someone would be compelled to pirate content and if it can be justified with little scrutiny as to the morality behind such actions.