Sneaky bf emotionally cheating

My bf started emotionally cheating on me with a girl on social media. He doesn’t know that I know but I have proof. I’ve heard people say you should just trust him, he needs privacy you should give it to him, don’t go thru his phone it’s not right and can damage your relationship. But, why does he get privacy when all he does with his privacy is use it in In appropriate ways. Once he started talking to this girl he changed his passwords too but I got to see enough before he changed it to call it cheating. He may not meet up with her in person I’m not too sure about that, but he definitely flirts with her and doesn’t mention me and hides it from me by changing his passwords so I personally consider that being disloyal. Please tell me what side your on.