Responses
He projects his action, not the seme as a person, he has guilt. Like "i should've done this and that" this can happens in our daily life too.
Small example is you witnessed your brother got food poisoning bc he didn't carefully check the expired date, so in some worry when you have a boyfriend that you fucked you somehow remembered the name experience and always make sure to check the date beforehand giving it to yout boyfriend.
Though I understand the reason why you're uncomfortable, probably bc of the age gap and "family found" dynamite
idk how anyone aren't finding this manhwa weird and, think of it like normal relationship. the seme is literally projecting his little sister onto the uke as "someone to take care of" like a little sister. and that would develop into romance later. if yall isn't feeling that weird, then idk