Point is Miss D ~ As a parent you either want to know EVERYTHING your child is doing or you want to know NOTHING about what your child is doing.
Frankly the way to sanity is the latter option. Impractical I know but it does make for a restful life. They do eventually reach an age when you are better not knowing ...... it is at that point that the little sods start telling you everything.
You will of course now be accused of breaching her privacy, violating her trust etc etc and you may drive the "affair" underground. What you will never do, unless you place her in a convent, is stop her seeing him or any other boy that takes her eye. Take the pragmatic view. Use this as the opportunity to speak of mutual trust, her good sense (in which YOU have implicit faith) and have a long talk about birds and bees, the ruining of a young life etc etc. Then get her to the doctors for the anti cervical cancer jab ASAP ~ my little "angel" managed to become "sexually active" before we had the chance to get that jab for her and she now does not qualify for it, having ruled herself out by one little bit of nooky.
When we accepted that we could not be with her 24/24 and that if she was determined to have sex with her BF (OK she is almost 16 but!!!!!) and that my ripping his bollox off would only drive them closer together we made damned sure she was on the pill. She then accused us, based on some spurious stats issued by the school in one of their "Rights without Responsibilities" lessons that by taking the pill for a prolonged period (ie starting it so early in her life), of almost certainly condemning her to future breast cancer I could have, but didn't, strangled her.
I tell you all this to demonstrate that you need to consider what you say ~ do NOT speak in anger ~ which I know you rightly feel, as did we.