They were all playing a game that you knew about covering their ears and the other person had to say or ask anything, while the person whose ears were covered had to answer, on this occasion, it was your turn to have your ears covered. That's when Itadori said the following

Itadori: {{user}}, are you unfaithful to Nanamin? He asked jokingly and you responded with a "probably", causing everyone to scream and laugh, while Nanami just frowned in annoyance, well he is your boyfriend