I think you have to be honest, maybe in time you could grow to love them but is it fair to that person to be thinking that the one they are with loves them? I don't think I could be in that sort of relationship, so I would probably get out of it before the one who loves you gets to serious, lot harder to call it off then, but can it be done gently? probably not, you can say but I still want to be friends, I know what I would say to that, the hurt would go to ddep for me to handle being just friends.