10 After this the Lord appointed seventy-two others and sent them two by two ahead of him to every town and place where he was about to go. 2 He told them, “The harvest is plentiful, but the workers are few. Ask the Lord of the harvest, therefore, to send out workers into his harvest field. 3 Go! I am sending you out like lambs among wolves. 4 Do not take a purse or bag or sandals; and do not greet anyone on the road.
(Note that these 70 disciples are not prohibited to marry).
Here James is not one of the 12 apostles:
4. James, the brother of the Lord, succeeded to the government of the Church in conjunction with the apostles. He has been called the Just by all from the time of our Saviour to the present day; for there were many that bore the name of James.
http://www.newadvent.org/fathers/250102.htm
So this may justify why in 1 Cor 9.5 there is a distinction between the apostles and the 'brothers of the Lord'': the latter were not apostles.
Now, Hippolytus counts James among the 70 disciples:
1. James the Lord's brother, bishop of Jerusalem.
2. Cleopas, bishop of Jerusalem.
...
etc...
2. Cleopas, bishop of Jerusalem.
...
etc...
http://www.pravoslavieto.com/docs/sv_ot ... _12_70.htm
The view seems to be confirmed by Eusebius:
One could perhaps say that the twelwe would be the first apostles, but not least were also the excellent Paul who was ''called an apostle'' and "James the Lord's brother", who is remembered as the first bishop of the church that was established by the Savior himself in Jerusalem.
(Commentary on Isaiah, 17:5)
If the distant Corinthians have to know 'brothers of Lord'' with the right to take wifes (1 Cor 9:5), then it is more probable that these ''brothers of the Lord'' were the same ''70 disciples'' sent by Jesus "to every town and place" (Luke 10), than these people were carnal brothers of Jesus living in Jerusalem.
It would make sense, for Paul, to use a rethorical argument to the extent that, even these 70 disciples, who are famous people in virtue of their abstinence from any pleasure, well, just themselves (!), can have the pleasure of a wife, whereas the poor Paul can't.
The title ''of the Lord'' should be derived from the same words of Luke 10:
The harvest is plentiful, but the workers are few. Ask the Lord of the harvest, therefore, to send out workers into his harvest field. Go!
Insofar James is sent by the Lord, then he would become '''Brother of the Lord', as co-worker in the his new creation, just as the Son was co-worker for the old creation.