People keep saying he's dense, but that's not the problem here.
This is like Komi-san if Tadano's self esteem was in the toilet and Komi actively cultivated the impression that she gives off.
And he doesn't even have a Najimi to whisper doubt in his ear.
"Oh, the pretty girl has invited me over to her house while her parents are away, and put on cat eats, and taken off her underwear. Could she be trying to drop a hint?"
"No, that's stupid, I'm stupid, she'd never stoop so low as to bother with me. She sees me as less than a dog and she's even told me so outright. She's just using me as a plaything to satisfy her sadistic tendencies."