Hi M.,
Wow! I hate that this stuff begins so early! My feeling is that your little girl needs to know what your desire is for her more than judgement about the girls "who participate" in this activity. This might be an opportunity for you to go to the bookstore and find a book which fits your belief system regarding social maturation, boundaries, sexual purity, dating etc. I believe that kids want to know "the why...behind the what." You might not find a book targeted to 7 year-olds but you can take what you read and water-it-down to become age appropriate. Some studies say that girls who know, see, feel that they are precious and special in the eyes of their father, are more secure to delay seeking male peer approval. Life is so full of things to explore and experiment with i.e. hobbies, sports, social and environmental philanthropic activites etc., that I don't know why any parent is allowing spin the bottle to be on the top priority list for a 7-year old. I'm so sorry that you have to confront this now but you are the best role model for your daughter and to show her how much you value her, you set her on a path to value herself.
L.