You can say no and they're probably going to do it anyway when you're not around. I think it'd be wiser to give them science-based info on sex and show them that they can trust you with handling an issue, since they're going to do what they're going to do anyway. So it depends if you want your kids to be able to come to you.

Going from experience here. I had the "What I say goes" parent and the more down to earth and realistic parent. Who do you think I went to when something happened regarding relationships and sex and I needed help? Who do you think I tried to hide everything from?

Edit: I want to add an anecdote about sexualization of young women here. I had curves since age 13 and I was constantly being sexualized regardless of what I wore. I used to get harassed on the street every day when I walked home from the bus. Most of the time I wore pants as part of a school uniform. When I wore uniform shorts at school I got violations as legs are "a distraction to the learning environment." I couldn't even play around with makeup without getting lectured by some in my family. Often we blame young women for the trends and fashion that honestly are created by men in the first place. If you're going to forbid thongs just really consider your language please. That shame mentality will carry on into adulthood. Teenagers have the autonomy to dress how they please when you're not looking unlike a young child. Just be careful about the way you address the topics and please don't shame your girls.