I agree with you Jay, I know for sure my daughter does not want her father to know about her behaviour, as she knows he would 'hit the roof' and not be able to cope with this knowledge. I want to keep pathways open for her to be able to talk to me whenever she wants too and I know also I can approach her too. I guess I'll just have to accept she's growing up and encourage her to be 'safe'. Thanks for all your support it has really helped me.

I think that one thing that you need to be thankful for is knowing that she has enough faith in you to trust you with that information. You also don't need to do anything to destroy that trust. If she comes to you with that she will come to you with anything.
Also, just because she had sex with a "long term" boy friend doesn't mean that she will have sex with this guy. I personally think that 20 is to old for a 16 year old. He is only in it for one thing! You might just try talking to her since she trusts you so much and tell her how you feel. It appears that she really respects you and what you think or she wouldn't have come to you in the first place. You may also want to put her on birth control if you haven't already. I don't really think that you have to much to worry about. If she was with the "long term" boyfriend then chances are she loved him and there was something there. That doesn't make her easy, that makes her human. If you freak out over it you can forget her ever coming to you with anything ever again. I would keep an eye on that 20 year old though. Don't be to pushy about her getting away from him though because that will just make her get closer to him. Good luck to you.