No, you're right, but I don't think they meant what you think they meant. They said she wasn't a virgin because she wasn't physically intact after the birth of Jesus. The Orthodox belief (affirmed by the Fifth Ecumenical Council) was (and is) that just as Mary conceived miraculously, she was also miraculously preserved physically intact even during and after childbirth. In other words, Mary was not just a virgin in the sense of never having had sexual intercourse, but also in the sense of remaining physically intact.
It's this purely physical virginity of Mary (her intact hymen) that was denied by Jovinian, and also by Tertullian, if I understand Tertullian correctly. Tertullian is a little confusing on the subject; he writes of Mary's womb being opened because she was married, but also makes it clear that it was Jesus who opened her womb. He writes (
De Carne Christi 23):
She who bare (really) bare; and although she was a virgin when she conceived, she was a wife when she brought forth her son. Now, as a wife, she was under the very law of opening the womb, wherein it was quite immaterial whether the birth of the male was by virtue of a husbands co-operation or not; it was the same sex [i.e., the male sex] that opened her womb. Indeed, hers is the womb on account of which it is written of others also: Every male that openeth the womb shall be called holy to the Lord. For who is really holy but the Son of God? Who properly opened the womb but He who opened a closed one? But it is marriage which opens the womb in all cases. The virgins womb, therefore, was especially opened, because it was especially closed. Indeed she ought rather to be called not a virgin than a virgin, becoming a mother at a leap, as it were, before she was a wife. And what must be said more on this point? Since it was in this sense that the apostle declared that the Son of God was born not of a virgin, but of a woman, he in that statement recognised the condition of the opened womb which ensues in marriage.
That is, Tertullian does seem to believe that Mary and Joseph had sexual relations, but he clearly believes that it was the opening of her womb
by Jesus that made her no longer a virgin. Whether she ever had sexual relations with Joseph is immaterial to Tertullian's argument.