i third that this is more about growing apart than anything else.
where gender is concerned - really is it all about gender? sometimes i dont think it is and we tend to slap on the 'gender eyes' when it really isnt.
it helps that at our house we have a lot of 'parties'. dd is an extremely social child and feeds off others. having people around her makes her happy.
her absolute best friend is a boy. but hse has her clicque of girls too. so when she wants to do girlie things she has a party for that without making her bf feel left out.
i really do think - this growing apart has everything to do with personality - not necessarily about gender. dd has grown apart from her few other boy friends she's had over the years. but her bf is beyond being just her friend. he is family and so are his family. personality wise they compliment each other even though their likes and dislikes are 180 degrees polar opposites.