Is it normal to fantasize and daydream about this?

Perhaps you could look at this problem from a different perspective. I does not have to come down to choosing between your current boyfriend and the other friend. Were I you, I would introduce your other friend into the relationship. How fabulous it would be having two guys inside you at the same time. Two beautiful erections pleasing you no end and all that delicious cum flooding into you. You would feel like you were in heaven.