Again, when taken in context we realize that Paul was not sharing his view on sex outside of marriage with the Corinthians in his letter. The Corinthians already knew sex outside of marriage was sinful and Paul felt no need to explain that to them. When taken in context and using your own statements about society at the time, we come to the inevitable conclusion that the sinfulness of extramarital sexual relations was not something Paul taught the Corinthians through his letters, it was something they already knew and had accepted.

Thus, even if we disagree with Paul, we cannot claim that the sinfulness of extramarital relationships was his teaching. There is evidence in his own writings that this was already an accepted tenet of Christianity at the time. The letter to the Corinthians wasn't written as if to people who did not already know this basic teaching, it was written as if to a people who already had a full understanding of what the Christian view on the topic was and that it was in conflict with the accepted moral standards of the time.

As a side note, I think you are being way too quick to jump to assumptions about what my beliefs are and how they differ from your own. It seems to me that our basic beliefs are very similar if not nearly identical. Don't let my desire to explore your point of view fool you into thinking that the entire basis for my belief is so different from your own. To make it even clearer, let me just state outright that I do not believe Paul and his writings to be the ultimate authority on Christianity either.