I didn't say he's stupid though, I said he seems to miss some obvious clues. Again, I realize that I have the advantage of conservation of detail working in my favor, and that the MC has actually spent days living and working and fucking his harem between one clue and the next, so they will not be as clear in his mind as they may be in mine.
The Fairy thing is a pretty strong example of conservation of detail: the author introduced this out-of-the-blue group of tiny people, and made sure we could get a lot of details about them, thus it must mean something important. The MC is being bombarded by the 24/7 news cycle, so what is and what isn't important may escape him.
However, the sheer risk of any part-elf whose parents are unknown turning out to be a half.elf (and therefore a roomba) should be pretty clear in his mind when he is so worried about his own child (with Lin) becoming somebody's roomba.
Blondie had a pretty strong opinion that Maria should not look into her parenthood. He should at least have had a vague sense of unease, and perhaps tried to convince Maria to wait a couple of days, while he teased some more details out of Blondie, who may very well answer vaguely that she heard stories about orphans turning out to be half-elves and being sold into slavery on the spot.
I can make up *some* excuses for this, but the fewer excuses I have to make up or the MC's behavior the better the story flows. Light SoD is pretty cheap, so handwaving some things is fine, but there is such a thing as the straw that broke the camel's back.
Take Lin's complete lack of cooking skills, for instance: she has *amazing* memory, and close to 300 years of either living on her own in the wild or working for human masters under her belt. How has she never learned how to cook?
That is actually lampshaded by the MC at the very beginning: 300 years and she never learned how to cook?!
We learn why later on: it was a sort of rebellion, possibly unconscious. When she says that she learned to cook because now she had good motivation, it implies that in the past she either had no motivation to learn or actually had some motivation NOT to learn.
Though frankly the entire thing with Lin going from "burns water" to "master chef" in three months or less makes me think that the entire "elves learn slowly" thing is blown way out of proportion. Maybe elves learn differently from humans, and human training doesn't work well on them, or perhaps the people who are trying to teach them treat them like slightly smarter monkeys and don't actually provide good training, or maybe Lin is just that exceptional.
But that is a thing I can easily make up excuses for - I just did after all. It can also be flipped into an additional example of unfounded prejudice, which is always good as part of worldbuilding.
The point is, having to make up *some* excuses is fine, you literally CANNOT explain EVERYTHING in a story. A main plot point depending on the MC missing a clue that was clearly laid out for the players to see, however, is pretty bad, because the players are now feeling railroaded.
Oh, and BTW, I keep saying "roomba" and "fridge", but let's not kid ourself: the actual corret words would be "realdoll" and "onahole". That Lin didn't get raped *once* in 300 years of slavery strains my SoD more than the drones theft. But sure, the author wanted Lin to be "pure" for us, so I'll take that as a gift from God (i.e., Runey) and not question it.