Prove it. Find me data showing what the teen pre-marital sex rate was before the advent of the pill and condom as common contraceptives here in America. Compare that to now. I'm willing to bet good money that the stricter moral and religious fabric of society then beats the pants off today's "they are going to do it anyway, so why try to stop them" attitude. As I've said before, that line of reasoning is a cop out. Frankly, I think the people who spout that are ones who just think teens should do what they want regardless of the risks or alternatives, so adopt that philosophy to justify their position.
I got this information from a variety of sources so excuse me if it's a little jumbled.
Modern birth control as we know it was introduced in the 1960's.
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The condom introduced in latex forms in 1912, and polyurethane in the 90's.
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These are a few studies I've gathered that conclude that a majority of teens have engaged in some sexual act. And that even more in their late teens have had sex.
This Source shows the following of the rate of sex of people up to age 19 or 20.
By age 20 77% of respondents had sex, 75% of these before marriage, and only 12% had married.
The rest of it is based upon how many people(not just teenagers have had premarital sex, while I don't think it's exactly what we're talking about here it does add to the reasoning that most people are having sex before marriage.
People aged 44, 95% of them had premarital sex.
People who abstained until after becoming teenagers, 81% had premarital sex before 44.
Women turning 15 between 64 and 93, 91% had sex by 30.
Between 54 and 63, 82% had sex before 30.
Shows that just about all Americans believe abstinence is unreasonable.
Another study more specifically focusing on the sexual behaviors of teens.
Source
Slightly more than half of American teenagers ages 15 to 19 have engaged in oral sex, with females and males reporting similar levels of experience, according to the most comprehensive national survey of sexual behaviors ever released by the federal government.
The report released yesterday by the National Center for Health Statistics shows that the proportion increases with age to about 70 percent of all 18- and 19-year-olds. That figure is considerably higher for those who also have engaged in intercourse.
So I don't know if this answers your questions about before and after contraception pertaining to teenagers. But it does show that there is a gigantic number of people between 15-20 having sex. And I haven't found anything saying this trend is diminishing. So what should we do? Make sure they're safe doing it. That's not a cop out, that's making sure that the majority who do have sex do not spread STD's and do not have unplanned pregnancies.
And if they want to have sex it is highly unfeasible to stop them unless you can keep an eye on them all day and night. It would be like trying to make sure the tide didn't come in by piling a human shield by the shore.