Translation
Now I say this by way of a concession, not by way of a command.
Paraphrase
But I am saying this part about going ahead and getting married as a concession, not as a command.
A CONCESSION
However, the things Paul has said regarding going ahead and getting married are not his main point, but rather a concession because they are weak. He will now get back to his intended purpose which is for them to not get married during the present situation they are in.