Tertullian c. 155–c. 220 ADOn the Flesh of Christ 23
For she gave birth, who gave birth from her own flesh, and did not give birth, who gave birth not from a man's seed; and she is a virgin so far as a man is concerned, not a virgin so far as childbirth is concerned. ... In any case she is to be called not a virgin rather than a virgin, by a kind of leap mother before she was wed.
Tertullian's ground here is the birth itself: Mary's womb was opened in bearing Christ, so he will not call her a virgin intact after the birth. That is an argument about the parturition, not by itself about later children; the Monogamy passage below takes the next step.