[Photographed by Ad Meskens, source: Wikimedia Commons]
Despite these problems, the Roman army tended to encourage such unions, since the children they produced could eventually be recruited to serve as well. This would especially have been the case where the army had settled down more permanently and soldiers were recruited from the local population.
The ban was eventually repealed by Septimius Severus, probably early in his reign (193-211), probably in part to keep himself in the army’s favour. However, he only lifted the ban on soldiers marrying citizens. A soldier who married a non-citizen woman would have to wait until he was discharged for the union to be legally recognized.
*The Republic may have been another matter. The earliest mention of this prohibition refers to the reign of Claudius, but it seems most likely that it was instituted under Augustus.
Campbell, Brian. "The Marriage of Soldiers under the Empire." The Journal of Roman Studies 68 (1978): 153-166.
Watson, G.R. The Roman Soldier. Ithaca, NY: Cornell University Press, 1969.
A letter from a soldier's wife, 4th century CE, Grenfell Papyrus - Diotima [Note: this comes from after the time period under discussion, but it's an interesting read nonetheless.]