that's called "a normal person" mode
That would imply actually acknowledging awareness of the social "normal person"
of the world he's transported to. Which MC has done ZERO of.
You'd think he'd at least read up (briefly) on Wikipedia the standards of norms and behaviors for males in a 1:39 ratio. Civil rights laws. Standard interactions. Cultural differences. History lessons/key events. Myths, legends, religion, and children's morality stories. Which would
QUICKLY reveal this world's lack of first-generation interrelationship prohibitions. (Even classmates cited it was common for males. And step-sisters, genetically speaking.)
Even casual speculation or introspection would reveal this. Then I'd enjoy seeing him grapple with the implications of how he has to pretend, in order to not get caught. (Amnesia is giving him a partial pass in-universe; but MC is doing nothing during his grace period.)
Many, many, JP isekai authors fail to acknowledge this. They carry intellectual/ethical "baggage" from this world into whatever story they write. Like bringing along Banzai Japan Rice into medieval Europe because it's convenient.
If isekaied into a world as Triangles, Circles, and Squares (Asimov) -- I wouldn't try to drag human cultural norms into it. In only one panel does he acknowledge his own lack of inhibitions/brakes; then immediately disregards it.