Yesterday I had shower thoughts about this game's story, and have come to a conclusion that I will take as self evident truth.
I fully believe the thing the PC is working towards, is being either the Demon Queen, or Venereae's husband/lover/booty call/ etc, and the entire plot of the game is a series of tests to find the human most fit to pipe whichever one it turns out to be. Here's why.
The overall tone of the game is very laid back and playful. Individuals can and do have real problems, painful memories, and issues, but on the whole, it seems like everyone's having a good time being in on the joke as it were. Hardly the sort of atmosphere you'd expect if there was some driving need to defeat the Demon Queen. Speaking of, she herself sets up the series of trials the Hero needs to clear in order to reach her, and does it in a way to make it not only possible, but feasible, to do so. She's effectively carving the path to her own defeat. All the dungeons (even that pit of suffering and mind numbing tedium that will not be named) have a feel of performance about them. It actually reminds me of Morrowind, where part of the main story involves going to different temples and playing out different historical events/myths as a way of fulfilling prophecies. So while the Sigils and the attaining thereof aren't useless, they are a known quantity in whatever prophecies surround the Hero and everyone is invested in making sure the chosen one gets them in a proper order and manner.
Then there's our alleged goal. Defeat the Demon Queen. Why? There's no war, she's not threatening to start up the war, and no one's even brought up the possibility of restarting hostilities between humans and monsters. (except our cute lizard waifu, but she just wants to fight to find her soul mate) Unless everyone involved in the story knows something we don't, and decided against all logic that informing the Hero of the urgency of their goal, there seems to be no actual reason to disrupt a mostly decent status quo that's slowly improving. If anything, if defeating the Demon Queen did have actual consequences, it could just as like make things worse. If someone giving her a good dicking down made her not the Demon Queen, then we could just as easily get a new one who isn't so on board with maintaining peace and letting relations heal. No my friends, our goal and presumed final boss isn't for any reason that can't wait while we fuck about doing all the side content we want. I can think of two reasons why.
The first possibility is the Demon Queen is horny, but has high standards. Luckily for her, there's a process already in place to find and shape the ideal fuck buddy. All that needs be done is ensure said fuck buddy eventually meanders their way up to the Demon Queen. Hence the trials, all laid out before us like an amusement park ride. We aren't actually questing or anything, just going through the set of tests that ensure we're the one who can satisfy our future wife. The other, is honestly mostly the same, but it's our Goddess herself who wants the D. That turns beating the Demon Queen to basically just another step along the way.
Now, there may be other reasons for this story to unfold. Some other goal that may be revealed after or en route to the Demon Queen's fight. But so far, I haven't seen anything to foreshadow a deeper reason behind it all. Meaning I'm only working with what I have, and can easily extrapolate. One final thought, though. It could be our quest isn't just to pipe the Demon Queen. Since everything right now is kind of alright, that may be enough so everyone just wants to keep the status quo. But, it could also mean instead of stopping a war, overthrowing evil, or anything like that. What we're about here, is speeding up the healing process of this world.
My thoughts on it go thusly. Things are alright, a lot better than they were. We're not killing each other, and a few people from both sides are able to live in peace on the other's turf. But we've already seen from a few monsters that there's old wounds, still not healed. Could be, that the generations since whichever DQ made the enchantment, people were content to let the smoke clear, live and let live. But this isn't a perfect solution, the monsters are effectively trapped there, and very few people are getting used to the idea of living alongside one another. So, perhaps everyone involved in the plot had, well, a plot. If DQ has an equal, a husband/wife who can truly be said to stand alongside her, that could be a potent symbol. For either side, the top monster having not just a human cum dispenser, but a partner would be a powerful sign that coexistence is a real possibility.