K.R.
Oh, isn't this another joy of parenting! I agree with what has been mentioned above. I would talk with your daughter and listen to her definition of a boyfriend. It will allow you to begin communication about this next area of development for her. I would also clearly let her know that single one-on-one "dates" are out of the question until she is 16.
Having been a teacher for 10 years in the middle school, I know that boy-girl friendships do occur. Some are more mature than others. If you tell her "no" she will have a relationship behind your back. I would advise that you listen to what she wants and determine what, if anything, out of what she wants, you agree with. How can you make it a situation that allows her to develop socially, yet be safe for her age and maturation level?
I have always encouraged parents to discourage single-focused relationships by encouraging group activities. And don't leave it to other parents to chaperone those activities, invite them to be at your house where you can monitor and interact with her friends. This will allow you to monitor the relationships she is developing, have healthy dialogue with her about what you observe and keep that door of communication open.
You will most likely find that if you allow these friendships to develop, evolve and go their own way, they won't go anywhere, especially when there is enough opportunity for healthy interaction within the context of a group.
Both you and your husband will get to model for the kids she has over what concerned, caring, positive relationships are in the process! Kids need all the positive role models they can get!