Is it true that there is cheating involved with the first couple??

I just finished vol 1 and I don't know if the cheating you're talking about happened within the volume. If it did, they barely dated and their relationship is quite a mess. They both knew that so when they talked about Mi kun's hooks up, it didn't cause any issues. Personally, I don't think it's cheating but since they are indeed in a relationship, it's true that there is cheating involved. And since the characs don't think it as a big deal, I think you should treat it as such as well.