I am feeling a bit too down to sift my notes at the moment.
But, I take it, that you are Catholic and perhaps speaking to non-catholics?
If so, the scriptural references are probably the most effective.
So, if I do a quick scan of the LXX and NT…
Genesis 34:31 uses it to refer to a possibly legal raping.
πορνη pornH (the H is Eta)
Genesis 38:15 uses it to refer to a harlot who was not really a harlot, but a woman Judah owed children to through his sons. πορνεην pornH[n]
This is backed up in line: 38:21 when the locals ironically note there was no harlot there… she took the child and pledge as her rightful payment.
In Genesis 38:34 the word is probably the same variant you are looking for by my guess, which simply re-affirms the last two passages.
πορνειας porneias, (from the child is conceived by porneias)
εκ-πε-πορν-ευκεν (hmm, an odd one, will need to look at later as it refers to Tamar’s standing for which she is to be burned (purified) )
In Genesis 34:15, The septuagint is different here, I think than the Hebrew, but the passage is talking about marriage to those not of the same religion. The standard text does not draw out the full nature of the contract. But the Israelites are obviously using guile in promising a marriage if the Pagans will MERELY consent to circumcision. The outward sign of their religion – when the cutting off is really to be of their heads above. In other words, by killing those outwardly circumcised (married in the covenant) since rape (of which the whole other tribe is “guilty”) is incompatible with marriage.
The circumcision is carried out by some of the priest/butchers of Israel. Hmmm, And Jesus says a true Israelite has no guile… I don’t know in what sense to think of that.
Perhaps this is enough of a start for you? although, I would encourage you to check some of these verses with a Greek & english copy (interlinear) of the Septugint (some online, some Libraries also have them) so you can verify what I have said yourself. The Greek is the same kind as was used in Jesus’ time, as the conquering government of Alexander had preserved the language for centuries – Much like the American Declaration is quaint, but still readable to Americans – and not so different as Elizabethan English (not our country). Of course the supreme court seems to be able to read all kinds of things into that declaration…
Scholars will be Scholars… a Thesis is a dime a dozen.
But, it does not appear to matter whether a marriage is implied or not. Premarital being the cover word for “we are going to be married” so let’s start now. But , as should be obvious, any woman can claim rape after the fact whether she consented or not – so at least that much ought to be an issue for those who are “test” driving, because not only do people change unlike cars after marriage from before – this “car” could invent a contract of its own…
I would say, since the word is used of foreign Gods as well as marriages, it would seem to imply any activity not sanctioned by the religion revealed by God to eradicate the sin of the Pagans. But, the minimum I could hold conservatively in a debate is that it refers to marriage rites outside those of God’s plan.
However, if you are trying to work with Catholics (gasp…I have met more than I thought existed) who profess the same reasoning – at least to the poorly catechized man one can encourage their restraint by emphasizing that the marriage rite has always been a matter of Law. (eg:the in-laws).
So that no one can marry without permission of the church – eg: that the “will” of the person which makes the marriage according to church teaching (after the vows) is insufficient if the spouses do not have permission. Jesus will not Join what God has forbidden to join. Therefore, it will always be considered fornication in the Law.
Does this answer the general query, or do you need something more specific?