In an article that is making the rounds on facebook (Where did the idea of Mary as "Ever-Virgin" Come From? by Kyle Roberts) discusses the origins of the belief Mary was a perpetual virgin. It is a decent summary of the issue, but he does make a blunder when he writes the following:
Theologians as early as Irenaeus and Clement of Alexandria affirmed the doctrine.
The reality is that Irenaeus of Lyons (130-202) did not hold to the perpetual virginity of Mary as a doctrine (let alone a dogma) and actually rejected the postpartum virginity of Mary. In Against Heresies 3.21.10 we read (emphasis added):
For as by one man's disobedience sin entered, and death obtained [a place] through sin; so also by the obedience of one man, righteousness having been introduced, shall cause life to fructify in those persons who in times past were dead. And as the protoplast himself Adam, had his substance from untilled and as yet virgin soil ("for God had not yet sent rain, and man had not tilled the ground"), and was formed by the hand of God, that is, by the Word of God, for "all things were made by Him," and the Lord took dust from the earth and formed man; so did He who is the Word, recapitulating Adam in Himself, rightly receive a birth, enabling Him to gather up Adam [into Himself], from Mary, who was as yet a virgin.
The phrases italicised (“as yet [a] virgin”) are clearly intended by Irenaeus to be taken as parallel to one another. Just as the soil of the earth was as yet a virgin (but only until shortly after when it was tilled), so also Mary was as yet a virgin before giving birth to Jesus. The direct implication is that she did not remain a virgin thereafter. While not explicit, it does show, at the very least, that the perpetual virginity of Mary was a doctrine unknown to Irenaeus.