I'm disappointed (but not surprised) at the amount of snarky comments at OP (like "WTF is wrong with you, of course there's nothing wrong"). I'm sure Tumblr is to blame for that. This is not an obvious thing to everyone. I work in the field of sex education and you'd be surprised at the amount of health professionals that are uncomfortable providing sex ed or sexual health care to teens who are sexually active. And to be clear, that shouldn't justify them not providing such care (but address that discomfort/prejudice and find more educational tools and resources to help them with healthy choices in their sex lives).
To answer your question, it's not a problem. Not all teens have sex (in fact, I didn't have sex in my teens) but there are those who do. You should also be aware of consent laws depending on your countries. For instance, my country has different consent laws between minors depending on whether they're straight/gay (which is fucked up). Does it mean I, as a professional or counselor, should report two gay teens to the authorities because of it? No, because you should also consider if that's an ethical decision to make and if that could trigger bigger negative consequences onto those teens (which is likely).
As others have mentioned, providing comprehensive sex education is an important aspect for teens and it will impact the way they make healthy choices in their sexual lives. Teens should also consent to whatever sexual activity they're having, be able to access condoms (external/internal) and contraceptives to prevent unwanted pregnancies. But sex ed is a great way to empower them: to let them know that they should be doing things that they want to do, not just what they think they should (or what they think their partners want).
I have plenty of resources on this matter if you'd like some more additional information (great tools from UE, UNESCO, etc).
