What do you think about me reading my boyfriend's old emails? I found out he tried to hook up with guys.
How does that make you feel?
I've already proposed to him and I plan on marrying him no matter what.
While it is not good that you went through his email and invaded his privacy, it is okay to be upset about him cheating. It is also okay to reconsider the relationship and its future. You don't want him to cheat after you are married.
He's the love of my life and we have something special.
That's good you feel so close to him, but you now have information that you need to process. Unless you're OK with an open relationship, you should think long and hard about what life with someone who hides their unfaithful behavior from you.