Arggggh! I just erased a very good start to my reply!
Here it goes again, albeit more abbreviated.
You should definitely look into the works of Joseph Campbell. He is the leading specialist on comparative mythology and spent most of his career analyzing recurrent threads in different culture's mythologies. His books can be a bit academic, so a good place to start would be his PBS video interviews with Bill Moyers, they are available at many Blockbusters, most libraries (try university's) and I've seen them a couple of times on PBS. His most famous work is Hero with a Thousand Faces that traces the cyclical path of the hero as he undergoes lesson-filled adventures that lead to an enlightenment that benefits both himself and his group. It's interesting to see how this can be applied to modern movies as well(ie: Matrix, Star Wars.)
That being said, I've given a lot of thought to your question and come up with a few different thoughts. One idea (not mine) is the concept of the collective subconscious. That there is a commanality to symbolic truths that is recognized by all people. That's the more psycological perspective.
The more esoteric one would be this: I believe that essentially, eveything in every universe is just a microcosm of the collective whole. We are vibrations of energy and collections of matter that operate in different visible ways at different wavelengths, but that all resonate back to the same source. And because we are from the same source, we are all one (corny I know). If we are just different views of the same thing, then it makes sense that we share thoughts, explanations. Everything is different, but only as light appears different when refracted through a spectrum.
And I don't think that is limited to our physical natures. I guess it does lead back to the collective subconscious but on a more spiritual level. We tell the same stories and we understand the same truths because it does not matter the distance and time b/t peoples, the truth remains essentially the same. And these stories of virgin births and floods, are such strongly ingrained allegories that have been vital to human's explanations of themselvs and their universe, that they transcend personal ideology and apply as universal truths.
The great flood is the great spring. Renewal of the entire earth. This one I think could even date back to a common remembrance of an actual flood that has been explained in every tradition at different times. Perhaps even our planet's beginning as water plays a part and our collective unconscious remembers "the great flood" more as the great beginning of life forms on our planet.
The virgin birth is a physical application of a god's otherworldly, divine status. He/She is not tainted by things of the body, does not cause pain (Buddha was born of his mother's side), and is born more of the thought/design of the gods than as a physical consequence.
As for creation stories, they are absolutely necessary for a culture's understanding of themselves and their origins.
You will find that genres such as creation stories, trickster tales, explanantions of natural phenomenon, etc are found in every culture because those are the things that early people needed to understand why the sky thunders, why some people act in trecherous ways, why in fact we are standing here, on this spot of earth able to ask these questions.