Why do you want to be with another guy when you love your boyfriend and want to marry him?
Sounds like your not ready for a committed relationship just yet. That's ok, good that you realize it now, instead of after a wedding. Just take it slow and talk with your boyfriend and tell him you honest feelings. Marriage is a job in itself and takes two people giving 100% of each other at all times. If you cant go to the table with that then stay in the seat your in now and work towards it. Maybe there are some things that this other guy does for you that you could suggest to your boyfriend and see if that helps. I wouldn't tell your boyfriend about feelings for other men, just that you want something different in your relationship with him. It is totally natural to feel for others we are from the animal kingdom, but its how you go about it that determines your moral being. Be honest with yourself and him at all times and you should do fine.