there's also a difference in philosophy going on here about what games (and hentai games) are or should be.
That's the long and short of it - I personally would never play a game resembling Thornsin, or really any of my most favorite H-games, if they weren't H-games. Don't get me wrong - some of them are actually mechanically sound, original and plain fun. I just feel that, unless it's in direct service to to the "H" part, it's a "wasted" effort. I feel stronger about that with H-games that go heavy on the story and dialogue.
I just don't think a permadeath system is suitable for the purpose, at least in this implementation. A common critique of many action-oriented H-games is that dissonance and conflict between wanting to lose to see the porn, and wanting to progress through the game (to be able to see the rest of the porn, usually). The more painless that choice, the better.
So yeah, a gallery should absolutely be a part of the game - if I'm playing to win, I'm never gonna
want to see those animations otherwise, when doing so brings me so much closer to having to do everything over again. One of the things I was trying to highlight with those examples is that all those games enable the player to go through many, many such steps towards failure before there's any real danger of being reset, and that's if we don't factor the ability to save-scum through each of them (though in AAL, only between missions). Apostle Angel Liebe pretty much lifts the map generation, floors etc from Rogue, but yeah I suppose it doesn't really count as one without permadeath. The others I mentioned definitely aren't, they're traditional linear H-RPGs.
Thornsin isn't helped by the compartmentalisation of enemy-types to each level, and having those levels be in the same, linear order - I think it'd be better if you had more freedom of choice of which "biome" to start in, and which to go to next - and reasons for and against choosing each of them. Eg. a mission-based framework.
It also outright prevents the player from easily losing to enemies they want, when they want, with many of the abilities and powerup effects being completely outside their control. At a certain point you just get so powerful the difficulty becomes trivial, but that conflict still exists until you clear the game. Just means that if you want to be efficient about seeing all the animations, you're probably best off getting to the last level and working your way back - which is incredibly time-consuming, with Thornsin's level & encounter design and progression.