Sure he should have known better, but he's pedestrian levels of naive and oblivious. You might be surprised to see what low self esteem can do to a dude.
The readership of this manga has the benefit of looking at the situation from the outside. It's not implausible for Masato to only have realized now.
Masato himself is technically an outsider, though, if that's to be a point of import.
I get what you're saying, and I imagine that at least some of the people here (myself included) are being dramatic. But I maintain that his line of "I thought I could keep living just as I did in my previous world, but..it really is a different world" is just silly, because it's
a different world and I have to believe that the average person, status of self-worth aside, would be a bit more mindful and attentive to how things operate on the off-chance that important details could mean drastic consequences for obliviousness.
It's not even the "relationship cues" part. It's the societal differences and the contrast of hierarchy, and it's not like he's not had time or resources to parse his situation and what he might benefit from changing about himself.
He didn't, and for better or worse, the chickens are coming home to roost.