I'm going to leave my fiancee for another guy.
Why do you want to be with the other guy?
He is more financially stable than him, is taller than him and has a bigger penis.
Those are superficial reasons to leave your fiance. If you truly feel that way then you should leave your fiance so he won't be hurt by you.
So what if it's superficial. I deserve to be happy.
Everyone does deserve to be happy, but think about how much you're going to hurt your fiancee by leaving him. It does seem like though it would be best just to end the relationship now instead of getting married and then getting a divorce. That would probably hurt him more.