You have my sympathies OP, my eldest is now 20 and my DC2 just about to turn 15, so I have navigated this road with my firstborn and am yet to navigate with DC2.
Honestly speaking, now that your DD and her BF have crossed that boundary and made their relationship sexual, they are unlikely to stop regardless of the barriers you put in place. I’m not saying you have to condone it, or that you have allow it, they are both underage and it’s such a tricky road to navigate, but you need to understand they will find a time and place to do this even if not under your roof, so that is something to keep in mind.
For us our DD1 was almost 18 and had been with her BF for a year so it was a bit different, we had already had the safe sex talk long before, but we refreshed it and simply discussed birth control options and helped her select one she was happy with and just made sure she was continuing to be safe.
The most important thing to do is to enforce the boundaries you set, whilst also maintaining an open line of communication with her, discuss contraceptive options if you haven’t already, select one and make sure she sticks to it as she’s choosing to do an adult thing that has very very real adult consequences, regularly check in with her and how she feels, and just make sure she knows that she can always come to you.
That's about all you can really realistically do, yes you could forbid them, you can put a multitude of barriers, obstacles and boundaries in the way, but ultimately they will find a way around them if they really want to, so the best and most important thing you can do is just make sure she is being safe and sensible and fully aware of all the consequences and concerns of a sexual relationship.