It is often argued that Isis was not a virgin because, in one version of the myth, she impregnated herself using the severed phallus of Osiris after he was killed and torn to pieces.... a 'miracle conecption'.
However, in another version of the myth, Frazer points out that Isis was impregnated with Horus:
"while she fluttered in the form of a hawk over the corpse of her dead husband"... a 'virgin conception'

[ Frazer, James: The Golden Bough, p422 ]

" The late form of the legend goes on to say that Isis fanned the body with her feathers, and produced air, and that at length she caused the inert members of Osiris to move, and drew from him his essence, wherefrom she produced her child Horus."

[ Budge, E.A. Wallia: Legends of the Gods, Chapter 5 ]

One must understand that 'virgin birth'& 'miracle brith' share a core mythological line and, in regards to mythological history, are one and the same generally speaking, so the birth of Horus in both accounts are applicable, not to mention the precident for such miracle conceptions is absolutly widespread traditionally.

As Carpenter points out:
"There is hardly a god whose worship as a benefactor of mankind attained popularity in any of the four continents... who was not reported to have been born from a virgin, or at least from a mother who owned the child not to any earthly father."

[ Carpenter, Edward: Pagan and Christian Creeds, Chaper 10, page 115 ]

As Massey points out:
"The mythical Messiah was always born of a Virgin Mother--a factor unknown in natural phenomena, and one that cannot be historical, one that can only be explained by means of the Mythos, and those conditions of primitive sociology which are mirrored in mythology and preserved in theology."


[ Massey, Gerald: Lectures, 1900 ]

As Joseph Mccabe, a Catholic Presist for a time points out:
"Virginity in goddessess is a relative matter...Isis seems to have been originally a virgin (or sexless ) goddess, and in the later period of egyption religion she was again considered a virgin goddess, demanding very strict abstinence from her devotees."

[ McCabe, Joseph : The Story of Religious Controversy ]

Some other sources on this point:

Carpenter, Edward: Pagan and Christian Creeds, Chaper 10

Massey, Gerald: The Historical Jesus and Mythical Christ

Doane, Thomas: Bible Myths and Parallels to other Religions, Chapter 7
Acharya S.: Suns of God Chapter 7