Ethan cheated and Tristan didn't, he was sexually assaulted by his "father figure".
But at the end of chapter 81, it looks like Tristan cheated cuz he was naked with that bite mark and now Ethan is pissed lol, serves him right for cheating tho. There was no reason for Ethan to do that at all. The author could have written another way instead of having Ethan resorting to cheating but okay(¬_¬)..
Also, I just realized the amount of characters having sex with the kids they literally raised is way too high in this manhwa