Banning sex from a hormone-addled adolescent is like ordering a monkey not to swing from a tree branch. While both acts might cause disobedience, the first can create havoc in the parent-teen relationship.
Parental prohibition is basically a control play, and the reality is, it doesn’t work. When children are little, they will find sugar even when their parents “censor” candy. Kids will find access to alcohol regardless of whether or not their parents forbid them to drink. And, teenagers will be sexually active despite their parents’ rules.