Ah yes, an open door, the ultimate and unbreakable curse that forbids a teenager for having sex no matter what.
The best thing a parent can do when their kids become sexually active is to buy them condoms. Many unsecure sex at that age is done out of pure embarrassment over going to a place to buy them or having their parents know what they are doing. They are going to have sex regardless, at least do your best so it is safe sex.