OP you've taken a great approach and done exactly the right thing.
Some of the more recent comments on this thread astound me. The posters sound like they live in a parallel universe to mine:
In my universe 15 year olds generally have a pretty significant sex drive- which although uncomfortable for their parents to acknowledge is a natural part of puberty. They may not be emotionally mature enough to cope with being sexually active, and it is of course not ideal- but that is a very separate issue to the fact that it is very normal for them to have an interest in sex.
In my universe 15 year olds can be cracking on with their studies, playing sport, chatting about Roblox, and also be doing things like OP's daughter. These things are not mutually exclusive- the presence of good grades, hobbies and sporting activity does not mean kids are not becoming sexually active.
In my universe it is toe curlingly awkward for 15 year olds to discuss this stuff with their parents. In fact most people of any age find it at least a bit awkward to discuss their sex life with their parents. For this reason it is wise to tread carefully around this stuff with our young people and to aim to foster a relationship where they feel able to come to us with worries or issues around sex without fear of judgement. They should know that awkward or not, we are a safe place especially if things go wrong- and shaming, judging, punishing, and being totally out of touch with reality means they are less likely to come to us when they need us most.
Unless you are actually locking your teens up they will be talking about things, and doing things that you don't know about. If you can't accept this then you do not know your children and are living in a fantasy land.