Honestly? Those YouTube Club members trying to force Mari to confess things is messed up. They're simply using Mari's attractiveness to get them more views! Everyone knows she's a queen bee around school.
Seki was right to put a stop to it. Since he knows a lot more about Mari than other people do, he knows they'll probably be asking more and more perverted questions she would never want to actually answer. So despite her not liking scary guys, in his desperation, he scares the YouTube club into stopping the broadcast.
Now their relationship will definitely change! Because Mari went to Seki because he was the quiet and kind type, yet now he's shown a scary side she doesn't like at all.
I'm guessing Mari's scared of people who give off a delinquent vibe because of the person who stole her mom from her dad. Her mom absolutely regrets having done what she did. And she keeps begging Mari to not be like her.