What's is the case for premarital sex being an instance of πορνεία (porneia)? This question is pretty much a spinoff of What does the Bible say about sex before marriage / premarital sex? because its current answer depends on the assumption that premarital sex is πορνεία (commonly translated "fornication"). Not all agree, so I think this assumption is better explicitly questioned.

I've heard some liberal Christians dispute the definition of the Greek word πορνεία (porneia), claiming it doesn't mean premarital sex at all. (An example of such an argument)

What's is the case for premarital sex being an instance of πορνεία (porneia)?



One of the major difficulties here is that just a dictionary definition doesn't cut it, because it's difficult to really know what the Greek word really meant 2000 years ago. The answer probably needs to either be a broad look at the Bible passages using the word, or an academic study on the word's meaning.

Additional information 2011-09-21: The following articles seem interesting, but I can't access the fulltexts. It seems from the publicly-available bits that Dr. Malina might actually have been the first to dispute the meaning of porneia.


Malina, B. Does porneia mean fornication? Novum Testamentum, 1972.
Jensen, J. Does porneia Mean Fornication? A Critique of Bruce Malina. Novum Testamentum, 1978.
The word porneia is often translated as "fornication". 


  1 Cor 7:2 (KJV)
  Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.


It's worth noting that more modern translations use "sexual immorality" here instead of "fornication" due to the confusion with that given word.

Looking at the English definition of "fornication", we see:


  Dictionary.com
  Fornication
  
  1.
  voluntary sexual intercourse between two unmarried persons or two persons not married to each other. 


Premarital sex is fornication; adultery is fornication; any type of sex where the two people are not married or not married to each other is fornication.



Strong's Concordance gives us the original definition of porneia.


  1) illicit sexual intercourse
  
  a) adultery, fornication, homosexuality, lesbianism, intercourse with
  animals etc.
  
  b) sexual intercourse with close relatives; Lev. 18
  
  c) sexual intercourse with a divorced man or woman; Mk. 10:11,12


This root word includes all forms of sexual acts that are not within the confines of  marriage (a heterosexual one at that).  The study of the word itself shows that anything and everything outside the realm of marriage is a sin.



It's also worth noting that this word is one of the root words for pornography.

Regarding your link
In this answer, I point out the clear and obvious refutation to link that you provided above.  They claim that adultery was a property rights violation against the husband.  However, they ignore the fact that raping a virgin was a property rights violation against the father.  Furthermore, if a woman became married and the husband found no proof of her virginity, he had a right to have her stoned for not being a virgin.  Clearly, premarital sex was explicitly forbidden.