The best thing that you can do is be real. And Honest with her, too many times, parents aren't wholey truthful with their kids and that drives walls between them. Explain to her that you know that this is something that she needs and its hard for you to let her, but you do because you know that she needs this experience. But on the same note, explain to her that you need to know that she's going to follow through on her word if you're going to let her have her freedoms. Try not to get bitchy sounding or bossy, just explain it to her like you really remember when you were her age.
Above all... Be very up front about safe sex and birth control, She is human and that's going to happen anyway, so you might as well start drilling that into her head too, especially now
You are not being irrational .. you're being a good parent.
You have certain standards for your daughter and should be proud of that. Now you need to consistently enforce them .. without having to listen to her "negotiations".
The way you acted as a child has no bearing on what you can expect from your child.
Don't let her (or you) give yourself a guilt trip over wanting to do some serious parenting. Being a parent isn't a popularity contest .. it's not always easy when you want to do it right.
They are your rules .. make sure she knows that's how things are going to roll. When she realizes that .. she'll follow them.
I'd be more worried that my 15 year old daughter has been putting her head up peoples butt's. But no I don't think you're being too harsh.