What would be a good counter arguments to this idea? Sex Before Marriage Is Not A Sin

I found this article that says that sex outside marriage is not a sin.

Some samples are


  Ruth and Boaz
  
  Really? Where does the Bible give a green light to premarital sex?
  Perhaps the most striking example is in the story of Ruth, though
  there are other examples as well. According to the Book of Ruth, when
  the recently widowed Ruth and her mother-in-law Naomi were faced with
  a famine in Ruth’s homeland Moab, they returned to Israel impoverished
  and with little hope of survival. Ruth took to gleaning in the fields
  to find food for herself and Naomi. The owner of the fields, a
  relative of Naomi named Boaz, saw Ruth and was pleased by her. When
  Naomi heard about it, she encouraged Ruth to adorn herself and
  approach Boaz at night while he was sleeping to see what would happen.
  Ruth took this advice, resting with him until morning after first
  “uncovering his feet” (in Hebrew, “feet” can be a euphemism for male
  genitals). The next day, Boaz goes to town to find out whether he can
  marry her, and, luckily, another man with a claim to Ruth agrees to
  release her. They do marry and together they produce Obed, the
  grandfather of King David.
  
  None of this would have been possible if Ruth had not set out to
  seduce Boaz in a field, without the benefit of marriage.


The Bible’s Contradictions About Sex

Another is David and Betsheba. It's not a sin because Batsheba is not technically married.

David and Bathsheba says it's not sin at all.

Yet, not having sex outside marriage seems like the most talked about rules in christianity second only to believing Jesus is God.

So what would be a good counter-argument?

Also is there reasonable reason to say that this is very open to interpretation and that the bible is simply not clear about the idea? Is it okay to say that, yes it's sin and no it's not a sin is well within possible interpretation?
Where does the Bible give a green light to premarital sex?

First of all, the original word used in the Greek Bible, πορνεία porneia, and the Hebrew Bible, zanah, does not exactly mean premarital sex. The word has broad meanings, which may or may not include premarital sex:


  πορνεία: properly, of illicit sexual intercourse in general.


Second, there are verses in the Bible that explicitly refer to premarital sex, and even the monetary price to be paid for doing so, because the issue was dealt with from the perspective of property crime because virginity was considered valuable in Hebrew culture.

Exodus 22:16-17 


  16 And if a man entice a maid that is not betrothed, and lie with her,
  he shall surely endow her to be his wife.
  
  17 If her father utterly refuse to give her unto him, he shall pay
  money according to the dowry of virgins.


Deuteronomy 22 states that if a man and woman commit adultery, they are punished by stoning. And if man rapes a woman who is pledged to be married, he is punished by death. The woman is “pledged to be married”, pointing towards the woman already being the property of another man. So rape, as shown in Deuteronomy 22, is not a violation of someone’s dignity, but a property crime, which can be monetarily compensated.

The "ownership" of a woman's virginity is also illustrated in Genesis 19:8:


  Look, I have two daughters who have never slept with a man. Let me
  bring them out to you, and you can do what you like with them. But
  don't do anything to these men, for they have come under the
  protection of my roof."


In 1 Corinthians, Paul writes to Corinthians who had practiced unlawful sexual relations as stated in Leviticus 18. 

1 Corinthians 7:10-11


  10 τοῖς δὲ γεγαμηκόσιν παραγγέλλω, οὐκ ἐγὼ ἀλλὰ ὁ Κύριος, γυναῖκα ἀπὸ
  ἀνδρὸς μὴ χωρισθῆναι. 11 ἐὰν δὲ καὶ χωρισθῇ, μενέτω ἄγαμος ἢ τῷ ἀνδρὶ
  καταλλαγήτω,— καὶ ἄνδρα γυναῖκα μὴ ἀφιέναι.
  
  10 To the married I give this command (not I, but the Lord): A wife
  must not separate from her husband. 11 But if she does, she must
  remain unmarried or else be reconciled to her husband. And a husband
  must not divorce his wife.


The problem is that the term  ἄγαμος (agamos), “unmarried” means widowed men (ἄγαμος is used for men, ἄνανδρος (anandros, lit. without man) for women.



From 1 Corinthians: An Exegetical and Theological Exposition of Holy Scripture.

Widows, widowers, and divorced people weren’t allowed another marriage because of the transactional nature of marriage. “Unmarried” didn’t mean people who never married, but people who were married and are no longer married. Although the widows and widowers wanted to marry, they couldn’t because it was illegal. 

So here Paul is talking about marriage. There’s nothing to indicate that Paul is talking about sex here. And certainly nothing to indicate the Paul is talking about sex in the way it is seen today — two consenting, loving adults in a committed relationship that may or may not be bound by the covenant of marriage.