The prohibition of premarital sex is not at all obscure in the Bible. For example, I would note the “one flesh” motif:

Matthew 19:4-6 (RSV) He answered, “Have you not read that he who made them from the beginning made them male and female, and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh?’ So they are no longer two but one flesh. What therefore God has joined together, let not man put asunder.” (cf. Mark 10:8)

This is obviously a reference to intercourse, so that becoming “one flesh” is clearly morally done after being married, not before. St. Paul expands the argument and ties it in with the notion of the Church and Christ:

Ephesians 5:28-33 Even so husbands should love their wives as their own bodies. He who loves his wife loves himself. For no man ever hates his own flesh, but nourishes and cherishes it, as Christ does the church, because we are members of his body. “For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh.” This mystery is a profound one, and I am saying that it refers to Christ and the church; however, let each one of you love his wife as himself, and let the wife see that she respects her husband.

The husband loves his wife because he is one flesh with her, just as Christ and his Church are one. This presupposes that sexuality is properly only within marriage. Thus, St. Paul offers a contrast of unlawful sexuality elsewhere, by noting how utterly immoral and improper in the Christian scheme of things, is sexual union outside of marriage:

1 Corinthians 6:9, 11, 15-20 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither the immoral, nor idolaters, nor adulterers, nor sexual perverts ... and such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and in the Spirit of our God. ... Do you not know that your bodies are members of Christ? Shall I therefore take the members of Christ and make them members of a prostitute? Never! Do you not know that he who joins himself to a prostitute becomes one body with her? For, as it is written, “The two shall become one flesh.” But he who is united to the Lord becomes one spirit with him. Shun immorality. Every other sin which a man commits is outside the body; but the immoral man sins against his own body. Do you not know that your body is a temple of the Holy Spirit within you, which you have from God? You are not your own; you were bought with a price. So glorify God in your body.

Again, quite clearly, premarital sex is immoral, and not just when a prostitute is involved, but any unmarried woman and man. It’s not fashionable to say this today (and certainly not “sensitive” or delicate), but so what? People sometimes need to be jolted into reality.

We must preach against sin from the rooftops. Christianity was never about being trendy and fashionable. Just because cohabitation is now ultra-fashionable as a new “norm” (i.e., “everyone else is doing it, so why not us?”) doesn’t change that fact. Truth is truth, and right and wrong are what they are, and are so eternally.

There are many biblical passages forbidding premarital and extramarital sex: whenever “fornication” or “licentiousness” or “immorality” or “sexual immorality” or “sexual vice” (various translations) are mentioned.

The problem, I submit, is not the unclearness of Scripture, but rather, the unwillingness to accept a difficult teaching and to counter worldly teachings and fashionable trends (post sexual revolution) in order to hold to biblical and traditional Christian teaching. 

I maintain that the prohibition of sex outside of marriage is quite sufficiently clear in Scripture. For anyone familiar with the Bible, it’s manifestly there, which is precisely why this was the uniform teaching among morally traditional Christians until we have come to this place now where it is fashionable in Christian  circles to question it. 

There are lots of clear passages condemning homosexual acts, yet lots and lots of Christians continue to butcher the Bible to find support for that. Young people are becoming overwhelmingly in favor of “gay marriage.”

There is no support for female clergy or priests in the Bible, and much about an all-male priesthood but does that matter? No. These two things are fashionable, so it doesn’t matter a hill of beans what Scripture actually says, or what the past tradition and history have held.

There’s plenty in the Bible about not murdering and not sacrificing children (I’ve collected it myself), yet almost all Protestant denominations have espoused abortion. Again, it was fashionable, so it was done despite what the Bible taught. 

The same dynamic is present in this debate. I don’t think most people who want to water down the true teaching are simply carnal monsters. It’s complex, as with all human motivation. Everyone understands sexual desires (that’s how God made us). People function within a framework. They “go with the flow.” This is what is so ominous today. As the zeitgeistgoes more and more against moral tradition, people jump on the bandwagon. Scripture becomes irrelevant. 

And if we Catholics agree that Scripture is so radically unclear on these issues, then we help support the tendency, even though we oppose it. We “condemn” those who will not consult Catholic tradition and the Church to believing the trends and the fad, by saying no one can arrive at the truth as regards the issue of premarital sex in the Bible by itself.