In the consulship of Constantine and Crispus [321] we have the following law [CI 5.26]

Nemini licentia concedatur constante matrimonio concubinam penes se habere. 
Permission is given to no one to have a concubine in his house during marriage.

Why? Why is Constantine banning married men from having concubines when it is something that has always existed in the empire?

Now, we see in Salvian of Marseilles, writing more than a hundred years later during the reign of Valentinian III remarking this in his book On The Government of God

the truth is more foul and loathsome by far—for certain men who have contracted honorable marriages take additional wives of servile rank, deforming the sanctity of holy matrimony by low and mean unions, not blushing to become the consorts of their slave women, toppling over the lofty structure of marriage for the vile beds of slaves

Now, I understand that what this does this essentially provide limits on inheritance regarding Roman citizens but could the government of Constantine not see that the citizens would just marry their 'maidservants' as Salvian says and therefore grant those children the privilege of filius legitimus?

What exactly is the objective here?