Constable has an answer in précis form which provides you with an answer to your question. In his interpretation, he assumes that the word this refers to the concession (or permission in the KJV) Paul had just made regarding a married couple who decide to abstain from sex (by mutual agreement, for prayer, and for a limited time).
Paul’s concession was allowing temporary abstinence from sex. The concession was not having sex. He did not command abstinence. He viewed regular marital relations as the norm. Paul was no ascetic who favored as little sex as possible. Abstinence was the exception to what was normal in his view.
I agree with Constable's interpretation regarding verse 6.
With verse 7, Paul concludes the section on married couples and moves on to the unmarried people in the Corinthian church. The two sections, one for the married and one for the unmarried, are not hermetically sealed off from one another; rather, they are connected to the general topic of sexual matters, introduced by Paul in 7:1.