The grooming term was kinda heavy. We could say he's indenial. He really thinks the mc is his tutor.
In another note, I also agree with you that someone who went(or even have physical relationship) with a person because they look just like their supposed true crush is a red flag. They're only in it for the body. It's not love, it's lust.
(I saw a manga like this with 9+ rating so I read it but I stopped coz the story is not my cup of tea. they're not lovers but the seme was just so horny and wanted to bed the uke but the uke kinda has uncomfortable past about it, so horny seme went for an app and saw someone who looks like uke. They became FB and enjoyed the sx with this slutty man who looks like uke. So I really don't believe him when he said he love the uke)
Haha i already shared a synopsis anyways just for a comparison. The manga just had a heavier atmosphere than this manhwa.

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I don't get how the 2nd ML mistook MC for the tutor or thought of MC as his replacement. Like, they are day and night different! Did he actually like him? It looks like he is trying to make the MC like the tutor. Grooming kinda. I think he was just obsessed as he met someone different. Puberty after all. If he genuinely liked the tutor, he wouldn't have tried to woo the brain cracked MC. He doesn't even care if they're different as long as he gets the face. Red flag behaviour.