Given how people are about romance and sexuality, you are concerned that you may ruin a wonderful friendship if the romance doesn't work out. One of the problems with people seems to be that when romance enters the picture, people change. They have different expectations of one another when they are romantically involved and they play different roles, often defining their respective roles by gender. Rather than relate openly and honestly, people in the throes of romance begin to hold secrets, and play the game of love. It is often the case that romance itself is an illusion.
People relate to more to the fantasy than the reality of their relationship.
Just the mere thought of having a romance with your friend has already changed your way of thinking about him. You are already plotting and thinking about ways to be less honest. Prior to thinking about romance, you would disclose just about anything to this closest person in your life. Now you are beginning to hold back. You can discuss your trepidation, as well as your affection.