Well that Changes every thing now... It sounds to me that your best friend is attracted to who ever your boy friend dates,
or he could be gay. and wants to keep girls away form your boy friend. (but thats unlikely)
This happened to one of my 2 good friends (who where best friends as well) one had a girl friend and one was best friends with his girl friend. and well the girl wasn't very fateful and left my one friend for the other. Now my one friend that got dumped for his best friend is in a new relationship with someone and she has a good friend that would be the someway. (that someone is me

) but I have no interest in her. But my friend (the guy) was getting paranoid that I was going to take her from him, but its all good now. WOW
anyways If you truly love your boyfriend then say with him, and... well try and not fall in love with your best friend. (ask him if hes gay

you never know.)