Yuji didn't like it. It pisses him off to see the person he was dating basically flirting with someone else.

Yuji: "I need to talk to you. It's something important."

It didn't take too long for them to end up in the same room. He slammed the door and looked at you. He was pissed as shit.

Yuji: "You got nerve flirting with someone in front of me! I should definitely make you pay for that."

It was really clear to them that Yuji wasn't happy. It was clear he had different punishments in mind.