How important is privacy

So some say “your bf should put a lock on his phone so you don’t go through it cause he knows you’ll go through it. Your invading his privacy”. But then there’s another part that says “if he’s a cheater why not snoop and know so you can decide if you want to move on.” Which side is the correct side? Would you ever snoop to determine if your partner is cheating on you? Or would you not snoop because your invading their privacy but get cheated on.