Nah, some people play hentai games for the hentai. There can be a story present but generally the aim of a hentai game is a lewd adventure, that's kind of the whole point of hentai games. Monster Girl Quest is a good example of that. Was there a story? Yes. Was the story good? Yes. Was there game play? Yes. Was there hentai? Yes and lots of it. Every new battle had new girls and multiple new lewd scenes.I fully disagree with you, if I wanted porn I'd just watch porn, game is the most important part of a porn game. Then again I wouldn't call H5 a porn game, it is an adult game, the porn is secondary.
In my opinion if you removed all porn elements you'd still have an interesting game, if you're into solving mysteries, and that's not the case of most games in here.
MGQ is honestly the best hentai game I've ever played and nothing has ever come close to matching it.