Now for the matters you wrote about: "It is good for a man not to have sexual relations with a woman."The punctuation, as it appears here, is very much in keeping with Witherington's interpretation: The part within quotes does not represent Paul's thought, but part of a letter Paul had received from the Corinthians, which he is about to respond to in the part of I Corinthians that follows. This interpretation is the current consensus of many scholars.
It is not, however, a universal agreement, and it is not even the interpretation of all English translations of the Scripture. Here, for example, is the same verse, as translated by the older NIV:
Now for the matters you wrote about: It is good for a man not to marry.Now, I should be clear that I'm not talking about the difference between the translations of the words (i.e., the difference between "to have sexual relations with a woman" and "to marry," both of which attempt to translate the same Greek phrase). I'm talking about those quotation marks which are present in the TNIV version, but not in the NIV. Most older translations follow the trend of the NIV, while many newer versions (including the conservative Holman Christian Standard Bible, lest anyone think that I'm suggesting some "liberal" interpretation) include the quotes.
To see how such a difference of opinion is possible, a bit of background is in order. The original manuscripts of these passages were not only written in Greek, but were written using all capital letters, with no spacing or punctuation. To illustrate the kind of confusion this occasionally creates, look at the following phrase in English: GODISNOWHERE.
Did you read that as "God is nowhere," or "God is now here"? With this phrase, sitting on its own, it would be impossible to be certain. Thankfully, we tend not to have such phrases sitting in isolation within the Greek manuscripts, but rather, we have complete texts that can provide context, and so scholars can usually weed through the lack of spacing and punctuation with a remarkable degree of accuracy.
Still, debates remain. In the case of I Corinthians chapter 7, the immediate context allows for either interpretation, but depending on which way we take verse 1, we will look at the rest of the chapter very differently. To use the traditional translation (without quotes), Paul is asserting his opinion that "It is good for a man not to have sexual relations with a woman." When Paul suggests, in the following verses, that "... since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband", he is then understood to be making a concession to the strength of the sexual drive. But if Paul were to have his way, we are left to understand, everyone would remain celibate.
But if we assume that "It is good for a man not to have sexual relations with a woman" is a quotation from an earlier Corinthian correspondance, the rest of the chapter takes on an entirely different meaning. Paul is actually arguing against the Corinthian position that all people should become celibate. Although Paul does agree that celibacy can be a good thing, he insists that such celibacy is not the norm. Paul asserts that most people should marry. Sexual relations (within marriage) are what Paul understands as normative, not celibacy! You can read Witherington's article for more about this particular case, including a little bit of explanation as to why this is the preferred interpretation (although this is not Witherington's intended focus).
This kind of textual difficulty is only one of many challenges that confront the Christian who seeks to be faithful to the biblical text. Yet so many people assert that "the Bible says" such-and-such on the basis of exactly these kinds of debatable texts, and accuse those who disagree with them of seeking to subvert the Bible, or of otherwise "not being a good enough Christian." A little more humility, honestly seeking the will of God, rather than assuming we've already found it, would be greatly appreciated.