‘But this I say by way of permission and not of commandment.'
While he gives this advice, he says, it is not something he has received direct from the Lord as an instruction. It is not found in the Old Testament or in the words of the Lord. But he is satisfied that he has God's permission to say it because He has revealed it to him. It is noteworthy that Paul does differentiate something direct from the Lord (1 Corinthians 7:10), and something which he has reasoned out for himself prayerfully before God with the Holy Spirit's guidance, and for which he then obtains God's consent. This last is important. What he says has God's consent. It is not just an unsupported opinion. But it makes clear that continual sexual relations within marriage is to be seen as the norm.
So even Paul, the recipient of God's inspired truth, demonstrates the respect the early church had for the actual teaching of Jesus, so that clear differentiation was made between His actual words, and teaching that arose from it.