Being misinformed due to the 'abstinence only' sex-ed. I wouldn't blame it on the teen, but on the education system, or the teen's parents who can't be bothered to teach birds and bees.
I was also taught: never carry a condom, because then you might be tempted to have sex. Instead of understanding that it's going to happen anyway, they teach that it shouldn't happen, so don't prepare for it.