I remember all these questions going through my mind, because I was raised with all the lines and the misinformation. The Christian reader expects the Bible to stop illicit sex! How else will we stop parents from having sex with kids, or women from marrying giraffes??!?!! (as Max Lucado puts it)
The commandment that Jesus lays down--to 'love one another'--might actually deal with those situations. I had to realize that Christian readers are trained to disregard the words of Jesus because they are describing an activity that reads as irrelevant. Love? That's weird and alien.
But maybe loving other beings around us is a difficult challenge that involves, for example, an effort not to harm. So "sex with kids" becomes a problem—under the rule of love.
There is no use of "porneia" in the Bible that doesn't relate to the spiritual identification of the covenant community. From Esau's "porneia" to the key example of Numbers 25—the defining example!—porneia refers to the improper "marriage" to another deity, if transacted sometimes through human women.
As to 1 Cor 5: there is no evidence that the "case of incest" has a son being sexual with his father's wife. This is a good example of Christian tradition making up facts and flouting actual Bible concerns. The word Paul uses is 'gynaika', woman. (Ancient Greek language has no word for 'wife'.) The reference is most likely to a father's concubine. The scholar Michael Benjamin Coves argues it's a clear reference to a play by Menander, "The Woman from Samos." I mention that here--
To have sex with the father's concubine is a problem throughout the OT—like Reuben and Bilah, Absolom & David's concubines, Abishag, etc. The problem is not "incest" as a crime involving family proximity, which wouldn't seem to be God's biggest concern (Abram and Sarai were half-siblings after all).
The problem is the effort to acquire leadership of the spiritual community using the means of a concubine who carries the leader's identification. Here is a nice little chat about that—
I do suggest that Paul is making a spriitual argument here—the problem is really Judaizing, as the father's "concubine" is seen as Jews. Not really "married" to Yahweh, but kind of an accessory. These are writers who take very seriously the use of female imagery for spiritual communities. The "Bride of Christ" for example was not just pretty talk. It meant you were Jesus' wife and should't be going to see other deities.