All of the answers so far seem to be by people with kids, so I'm gonna answer as a childless, 21 year old woman who was having sex at age 16. First and foremost:
There is no way that you can stop kids from having sex.
If you ban sex in your house; they'll just go somewhere else to do it. If you don't tell your kids that using contraceptives is important to avoid pregnancy and STDs; then you kid will get pregnant/catch an STD.
I know many people who were more responsible at 18 than some 40 year olds I know. Experience does not equate to responsibility.
At 16 (at least in the UK) your child is an adult and is viewed as sexually mature enough to make their own decisions. If you try to prevent your child from making these decisions, then all you are saying to them is that you don't trust them and that you don't see them as responsible.
You cannot ever prevent someone from having sex, just by banning sleepovers in your house. All you can do is educate your child from a young age on how to stay safe and how to make responsible decisions, then you can rest assured that when they are having sex they won't be doing it behind your back and they won't be doing it in an unsafe way.
My parents allowed me to stay over at my boyfriends house when I was 16, they knew what was probably gonna happen. They emphasised to me how crucial it is to be safe and I was encouraged to do my own research on anything that intrigued me. As such I have been having responsible, safe sex for 5 years and I am someone who is confident sexually.
Repressing the (completely natural) desires of your 16 year old is invasive and can lead to much worse damage in the long run. Hiding sex from them as if it's 'only for us grown-ups', will just mean that as soon as they are out of your grasp the first thing they will do, is what they were never allowed to do at home.
Educating your child is the most important thing you can do. On how to have safe sex, on how to respect their partner (i.e not forcing them if they're not ready), telling your daughters that they don't need to do anything just to impress someone, telling them about STD's etc etc etc All of this is far superior than even refusing to admit to your child that sex even exists.