Why couldn't Augustine marry his concubine?

From his late teens to his early thirties, Augustine was in a monsgomous relationship with an unamed woman. He was forced to give her and their son up when he got engaged to another; he did not ultimately marry, but never returned to her, though in his writings he still comes across as a lovesick teenager.

In the Roman Empire concubinage was an accepted alternative to marriage, where there were impediments to marrying. Problem is, none of them seem to apply in Augustine’s case.

Has it ever been confirmed what exactly the impediment was?

Well, for one thing, after he gave her up, she took a vow of celibacy, so there was that. But also, Augustine was the son of a local official from a prominent family, and was himself an ambitious teacher of rhetoric with political ambitions. Marrying her wouldn’t have been socially acceptable and would have ruined his career.

Her social status is never mentioned, so is that a fact or just speculation?