I will have to respectfully disagree with the person that wrote "there should be no secrets between spouses". A wife not telling her husband about their daughter having sex (if the mother found out in a confidential conversation with her daughter) is not about keeping secrets. It is about keeping a relationship built around trust intact with her daughter. I speak from experience as this happened with my mother telling my father about a conversation that we had about my sexual activity. At the time my dad and I did not have a relationship that included conversations about sex (I was his little girl and knew that he would not take the news well) and when my mother told him I felt so much betrayal that it took a long time for me to tell her anything that was personal again. The reality is that our kids have different relationship with each one of us and treating them with respect as to what they feel comfortable telling us is the only way that we can harbor a open and honest relationship with them. I would highly recommend that you encourage her to tell her father herself, offer to be with her when she does it and even ask her if she minds if you tell him but I would take her lead as to whether or not she wants him to know.

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.