It's just like doting fathers who doesn't want to be apart from their "daughter" as per say, It's not that deep, it's just an overprotective parents, as an asian, I get this a lot since my sister's bf took like 3 years before he's seen as approved by my parents, I mean he's a good guy, but warming up to the parents aint easy. It's also like entrusting the one you've raised into another person
They knew Kana since he was in diapers. There is no "warming up" to him needed if he was not good enough, it would have been obvious since a long time ago. They had no problems with the two being close friends, but when romantic feelings are involved, Kana is suddenly not good enough to date? If he was such a bastard that they do not want him in a relationship with Ritsu, then he was too much of a bastard to be his friend in the first place. And yet...
Ngl, I can't get over how weird the adults' reaction is to Ritsu and Kana liking eachother. They knew the kids since they were babies, watched them grow up, they know exactly what kind of people they are, yet they still act like Kana is some dirty old man who wants to take their daughter's virginity and then abandon her pregnant... Like, chill. Kana isn't a bastard in it just for the sex, and Ritsu is his own person, not to mention that he isn't even a girl, so some kind of misplaced misogynistic "she is weak and needs protection" isn't even aplicable here.