I guess the question ou should really be asking yourself, is do you think not allowing her to take the pill, will really stop her engaging in sexual activity?
Generally it doesnt work that way, so surely it is better to allow her to go on the pill and have protection from pregnancy, but sit her down, and have a grown up chat as well, explaining why you are allowing it, but that sex sint something that needs to be rushed, and you trust her do do the right thing and be sensible and take precautions when she does think it is the right moment for her.
There is nothing like crediting a young adult with being a responsible adult, to make them feel responsible. Generally banning them from doing something makes them do it more..
My mother followed a similar tack with me, and told me she would prefer me to bring boys home than feel i couldnt- and guess what. I never felt the urge to take her up on the offer, and took my time..
Good luck!