Go for it!
My best guy friend asked me out. I said no, but I've been thinking about it a lot. I don't want to make the wrong decision. What should I do?
Unfortunately, we tend to evaluate decisions based on the utility of their outcomes.
But the analysis of the risk and value of the outcomes of any decision are made at the time of the decision and for the most part don't involve life or death---you can usually modify whichever path you first take.
As John Boynton said, "Go for it."
By the way, this happened to me once. It got hot and heavy for about 2 weeks. Lot's of fun, but we decided to end it and keep the friendship.
Sounds like you already made your decision here so stick with it and don't start double guessing yourself. You had your reasons for saying no so a voice inside you was telling you not to or you would have accepted. One thing everyone must learn growing up is not to double guess ourselves as our imaginations will come up with a million reasons why our initial answer was wrong when in all reality it really is the perfect one for the situation.
So we must learn to live with our choices and move on whether it was the right or wrong one, and let it go. As for your friend, if he truly really wants to go out with you, he wil try again and again so maybe someday in the future youll feel different and accept, especially if hes really interested in you, or maybe one day youll ask him out. Good luck