It was when I was 17. Got together with that girl after a week we met each other. Being pretty naive, I needed safety and security in the first year of my boarding school in Norway, alone and apparently not knowing anyone (I had been living in Hong Kong all my life before that)- and you know those crazy little impulses that, when someone is being extra nice to you when you feel helpless you easily fall for that person? That's what happened to me- then after a week we got together it was her birthday and she was drunk- being Finnish, it seemed like getting as drunk as you can on birthdays is an obligation. I had a few drink with her but sober enough to remember what happened that night. Then I had to take her to bed and it happened. It was good and all that at that moment then it turned into one of the biggest regrets of my life though. Around 3 months later we had a break and being so close to home she went back. After that week of break she came back and I found out she cheated on me. Somehow I tried to forget about this and try to stay with her but it turned out to be plain bad
edit: No, I don't have anything against Finns. One of my best friends is a Finn.
Sorry for the extra bit of information. Just that the topic reminds me of this.
Slightly off-topic: I am the first person whom my current girlfriend slept with. But once we were talking about me being the first person she has ever slept with (months after) and she was like 'yeah what's so important about who's the first one?' I just feel weird when I heard that. :-\