Explaining why a man needs a wife, doesn't mean an unmarried believer will have sex outside of marriage.
Let us look at the following verse again:
"Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman. Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband" (1 Cor.7:1-2).
If doing that sin was impossible for a Christian then Paul wouldn't speak of avoiding it. You can't avoid something that can't happen. And here is what Paul tells believers about that sin:
"For this is the will of God, even your sanctification, that ye should abstain from fornication" (1 Thess.4:3).
Do you still deny that sinning defiles a Christian?