You were very upset, which is understandable.
This is a very important moment when you can build your relationship with her and ensure that she is more open with you in future, rather than sneaking around and doing dangerous things.
Rather than simply punish her I would be wanting her to have a very deep, honest conversation with you about why she made the decision to allow it to happen.
Start by empathising; explain that you have experienced those intense emotions so you know how big it feels, and that their drive is a natural human desire; nothing wrong with wanting to explore another person. The problem is that their decision was not safe, in any way.
Our kids care what we think far more than we believe they do, and building your connection with her by doing this will be a much more powerful deterrent than simply punishing her. You want her to consider the damage to your relationship more than her feelings for this boy.
Ask her what she thinks might be the real reason you reacted the way you did, ask her what she thinks the issues might be. Ask her if she’s discussed with him what they will do if she gets pregnant. (This is to really make her stop and think) Will she keep the baby? Will he look after her? What protection was she using? Does she know she can get pregnant from heavy petting?
I would give him the choice; either you tell his parents immediately or he sits down to discuss it with you beforehand and you will think about the way this is framed. Incidentally, parents of boys often don’t take this as seriously as they should.
if you can discuss it with him ask him how planned it was, how he thinks it might have impacted your feelings about him, how much trust and respect he thinks you should show him now.
Often kids don’t stop to think about the consequences of their actions, and they think it’s just about them; not their relationship with the people around them.
I would have a very calm conversation along these lines then ask her/him what they would do in your position, given what the issues are.
Your end goal needs to be to get their thinking brain engaged because just having penalties won’t work. Talk through the behaviour you expect from them and what your rules are. Ask them if they feel your viewpoint is unacceptable or unreasonable. You want agreement.
Based on what they say you can then decide how much you intend to restrict your daughter and the time she spends with this boy.
Here’s my blog on underage sex
https://www.teenagersuntangled.com/blog/it-might-be-illegal-but-when-did-that-ever-stop-a-teenager/
and another on setting consequences
https://www.teenagersuntangled.com/blog/whats-going-on-when-our-boundaries-and-consequences-dont-seem-to-work/
Good luck!