The latter, with respect to the Jewish culture they were speaking to at any rate.

Much of the New Testament was not aimed at people with a Jewish background though. It was aimed at Greeks, Romans etc Did the Greeks and Romans also frown on sex before marriage ?

Well now you're making an argument from silence.

No, I'm just pointing out that those Christians that lecture teenagers on the fact that sex before marriage is evil have no evidence to base their "rules" on. Yes the Bible says lots about how precious and special about marriage, but it doesn't say that this special and preciousness is specfically to do with sex. There is a lot more to marriage than sex after all.

There is plenty of evidence in the Bible to allow you to preach that adultery is wrong, but I see no justification for Christians that rile againt pre-marriage sex and claim that there is Biblical backup to their claims.