Hi, if we are being fair here, he was upset because she decided that sneaking Christian into their home was worth more than attending his championship game. They shared an unspoken tradition of always attending each other's games and sporting events. He was already hurt when he noticed she had begun pulling back due to Christian, someone whom he disliked. His hurt was further amplified when he realized that she chose to do it on the day of his championship game. So when he went inside his home, heard them laughing and rhythmic thumping immediately following that, he most likely assumed that she was sleeping with him. And if we are still being fair, he is in the right for assuming that. I mean, when I was her age, I wouldn't go to such lengths to invite a guy over to simply watch movies and joke around. She had intentions and likely got cold feet about it when Christian started taking initiative.
While I don't like how he handled things that time, he was in his right to act that way and assume those things. It was an important day for him, she knew about it(showed by how she invited Chris over because she knew her parents would be at MC's game), and decided that taking that big step with Chris was more important than supporting MC on one of the most important days of his high school years. I'm not saying his reaction was completely ok, but can you blame him?
P.S: English is not my native language, so there may be some errors in there.