The thing is, though, my parents would have been utterly disappointed if I'd come home pregnant as a teenager. They didn't let me have a boyfriend to stay over in my bedroom until I was 18.
I still lost my virginity at 15, and unlike OPs daughter, it wasn't to someone I knew a long time, it was to a virtual stranger. To be fair, he was a decent enough guy and it wasn't exactly a bad experience for me, and no harm came of it.
I was having casual sex by the time I was 16 and lying to my parents about where I was (potentially putting myself in danger). Nothing bad ever happen, and my parents never found out what I was up to. As I got older, they relaxed, and we went back to having an open and honest relationship.
I think it is very naive to think that just because a parent tells you to wait, you will wait.
Whilst yes, it is technically illegal, the law does state it "does not exist to prosecute teens of a similar age experimenting together".
In this situation, I believe your daughter would have probably had sex regardless, and she might have lied about where she was in order to do it. I think once you start lying, it becomes easier and easier, and it can lead to potentially dangerous situations.