No he never did.
But why are people saying that he went on a date with another girl from his school tho?
No, the thing was that Yamato and Megumi won a contest from school and the prize was a date. Even Mei knew about it cause Megumi told her first if that was ok. Though Yamato didn't want to, Mei agreed so as to move forward and build trust in their relationship . But Yamato never stopped thinking about Mei and Megumi realized that so they ended the date earlier and he ran to see Mei right away.
Now, I need to know what happened. Did Yamato really cheat on Mei? I heard he went on a date with another girl. It can’t be true right, hahaha. Mei is so perfect and precious. How can he do this to herrrr??? I need to know everything that happened right now. I’m really going mad. I thought they were the perfect couple, made for each other and all. I don’t think that it’s normal to be attracted by someone else to the point where you go on a date with them. Like wtfff.