The idea of Krishna as born of a virgin is actually an extension of the idea that his was an "immaculate conception". This notion was re-introduced in modern times by Acharya S in her book "Suns of God". I haven't looked into it recently, but I know there are plenty of Hindu scriptures that mention how Krishna was an immaculate conception. In one account, Devaki got pregnant by eating a fruit. In
another contradictory account, Yogamaya, Krishna's
internal potency, transfered Anantadeva from the womb of Devaki to that of Rohini. After that Vasudeva meditated until Krishna "appeared in his heart" and in the heart (womb?) of Devaki.
I also found these references:
That Devaki was "chaste" -
Srimad Bhagavatam (10.3.43): Devaki is specifically called "chaste" : "O supremely chaste mother...."
That Krishna was an immaculate conception-
Srimad Bhagavatam (10.3.17): "You never entered the womb of Devaki; rather, You existed there already."
There are as many references to both Krishna and Devaki being mythical characters, as there are of Krishna being an incarnation of Vishnu. The fact is, these tales are replete with many fabrications, and contain parrallel interpretations. It's like one of those books where you get to decide how the story progresses by deciding between alternatives. In the context of understanding where the biblical myth of Jesus (a relatively recent messiah myth) came from, historians have pointed out the existence of immaculately conceived gods in multiple cultures, suggesting that these myths may have had one or two origins and then moved around through word-of-mouth till writing was invented and the now-different and localized forms of these myths were codified as divine scripture.