h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I am pretty certain that Krishna was not born of a real or metaphorical virgin. He was the 8th child of a well wedded woman, who was imprisoned by her brother.

Similarly, the Gautam Buddha was pretty much a prince before renunciation of his title and his riches. His birth has no virgin twist to it.

The reference to these personalities/incidents seems flawed. It is also possible that these stories have been subjected to some manipulation.

Adonis as the son of Ishtar raises similar queries. Look at the incidents which culminate in the making of the Bull of Heaven - there's a catalogue of Ishtar/Astarte/Astaroth's sexual adventures which even Hello! or the News of the Screws would be happy to rake over.

Blast! missed this bit!
Yes, Siddartha has almost always been accepted as having human parents, before becoming the ascetic Gautama, and finally acheiving enlightenment as the Buddha. There may be odd sects who ascribe a parthenogenetic origin, but a religion without a personal god at the centre isn't an obvious source for this kind of myth.