Um...yes you absolutely can tell your boyfriend that you found this out. Even if it is by reading his emails.
And do you know why, honey boo boo?
It's because I'd assume you'd want to break up with him. He's living a lie. Which is his prerogative. But he's lying to you and sleeping with other people. Are you really trying to salvage that relationship to the point where you're worried that he might break up with you once he finds out how you learned of his indiscretions?
If you confront him, he will know. And he needs to know. Because he's cheating on you. And with men. He is actively cheating on you and living a double lifestyle. If you have decided that this is the man you want to be with, even despite this info, then by all means keep it to yourself. But if you realize that you are better than that, then let him know and kick him to the curb.