But if any man think. — Here the writer turns to the duty of parents, and there is a further explanation to such that the previous expressions are not binding commandments, but apostolic advice. If the case arises that a parent thinks he would be acting unfairly towards his unmarried daughter (i.e., exposing her to temptation) by withholding his permission for her marriage, he ought to do as he feels inclined — i.e., let the lover and his daughter marry.
Let him do what he will. — This sentence does not — as it may at first sight in the English appear to do — imply that he may consent or not, and whichever course he adopts he does right. It is implied, in the earlier part of the sentence, that he thinks he ought to give his consent, and therefore that is what he wishes to do. Let him do that which he so wills, says St. Paul, and he need not in doing so fear that he does wrong.