She wants to have sex?! were just friends!?

Ok so today is my 18th my best friend is a girl she is 19 since were both of age now she want to give me her virginity and to take mine? i really like her and she is attractive but she is doing this for the sake of a promise we made each another when i turned 15 after her first boyfriend dumped her because they... show more Ok so today is my 18th my best friend is a girl she is 19 since were both of age now she want to give me her virginity and to take mine? i really like her and she is attractive but she is doing this for the sake of a promise we made each another when i turned 15 after her first boyfriend dumped her because they couldn't have sex (because she was underage he was 18) at the time we thought it would be a great idea to give one another each others virginity because we really do care for one another and at least we wouldn't regret it, it do sent seem that way now and it never really did i don't know what i was thinking but she is determined to be my first and she wants me to be hers so she wont regret it but i think she wil and i don't want her to get hurt ....how do i explain this to her?
Update: i have told her how i feel now and i think she understands she promised she wont ask me about it, thanks for your help
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