You just made the claim that society in Corinth was filled with promiscuity. Thus sex outside of marriage was considered normal in that society. Yet when Paul wrote those words, he didn't feel compelled to explain why having sex outside of marriage is sinful. So, if you want to take what he said in context, how about the fact that it is clear from reading that letter in context that no explenation was necessary because it was already a well known tenet of Christianity that sex is for married couples? Were it not an already established Christian doctrine, Paul would have had to explain to the church in Corinth why they should not be having sex outside of marriage. Paul doesn't explain it, he assumes they already know and doesn't bother listing sex outside of marriage as a viable option despite the fact that it was the most common and socially accepted alternative in that community. So, it isn't just Paul then, but rather it is the established doctrine of the early Christian church, the very doctrine taught by Jesus' own disciples, those given the task of setting up his church on Earth.