Did all early Christians teach that Mary was a perpetual virgin?
It is true that the majority of Christians after the Nicaean are taught that Mary was a perpetual virgin, however prior to Nicaea there were two notable Christians that seems to contradict the idea, these were Tertullian and Hegesippus, both from the 2nd century. However, there were even later theologians who denied the idea, but they came into conflict with other early Christians.
Hegesippus 110 - c.180 AD
"There still survived of the kindred of the Lord the grandsons of Judas, who according to the flesh was called his brother."
according to the flesh" is a term used to mean genetic relationship
Hegesippus also distinguishes the Greek words "anopsis" (cousin, kindred) and "adelfos" (brother)
James, the Lord's brother, succeeds to the government of the Church, in conjunction with the apostles.
When they were released they became leaders of the churches, as was natural in the case of those who were at once martyrs and of the kindred of the Lord. And, after the establishment of peace to the Church, their lives were prolonged to the reign of Trojan.
Tertullian 155 AD - c. 220 AD
She who bare (really) bare; and although she was a virgin when she conceived, she was a wife when she brought forth her son.
On the Flesh of Christ
Helividius 383ad
Saint Victorinus of Pettau (died 303 or 304)
Feeling himself to be a smatterer, he (Helvidius) there produces Tertullian as a witness and quotes the words of Victorinus bishop of Petavium.- The Perpetual Virginity of Mary (Jerome)
Jovinian (died c. 405)
Apollinarius died 382
The Panarion of Epiphanius of Salamis: Books II and III. p. 616. As though they had a grudge against the Virgin and desired to cheapen her reputation, certain Antidicomarians, inspired by some envy or error and intending to sully men's minds, have dared to say that St. Mary had relations with a man after Christ's birth, I mean with Joseph himself. And as I have already mentioned, it is said that the claim has been made by the venerable Apollinarius himself, or some of his disciples.
Though Josephus wasn't a Christian, he wrote about Christians, Josephus in his writings always distinguishes the words "cousin" and "brother".
Flavius Josephus 37 - c. 100
Festus was now dead, and Albinus was but upon the road; so he assembled the sanhedrim of judges, and brought before them the brother of Jesus, who was called Christ, whose name was James, and some others, [or, some of his companions]; and when he had formed an accusation against them as breakers of the law, he delivered them to be stoned: but as for those who seemed the most equitable of the citizens, and such as were the most uneasy at the breach of the laws, they disliked what was done; they also sent to the king [Agrippa], desiring him to send to Ananus that he should act so no more, for that what he had already done was not to be justified