While I agree with your analysis, the biblical injunctions to marital faithfulness are directed at the church---i.e. at the people of God, not at the broader Gentile world. When the church/people of God stray from gospel faithfulness to Christ, that's spiritual adultery (c.f. Ezekiel 16).
However, Paul seems to assume the Gentiles will of course live like that---see his repeated refrain to Christians that they WERE like that until they put on Christ. Also, the history of early Christianity shows that rather than sleeping around, they practiced a counter-culteral monagamy: they were said to generously share their table and possessions, but not theirbeds or wife.
I don't see the New Testament condemning a pornegraphic pagan culture so much as calling Christians to faithful witness within it---all the while calling pagan Gentiles to Christ, trusting that once they were believers chaste sexuality would grow as they abided in the vine.