This isn't worth engaging in a fight about, and I'm not going to. Except I did interject myself so I'm going to do a jerk thing here: say my piece and walk out this thread's door.
I think there's no problem or offense if you ask people to speculate on the sun sign of a mythological figure - or, if we prefer, a hint of a figure stretching so far back in history we experience her today as primarily mythological. What "sign" was Achilles? What "sign" was Moses? What "sign" was Confucius? Could be fun.
But recognize that the only way such a question can be an engaging, educational investigation into astrology - or Christianity, in this case - is if we approach Mary as an archetype and consider what zodiacal sign seems to fit with that archetype.
The offense - and frankly, really, the dumb thing to do if you want anybody to discuss it with you - comes with scoffing at the first person to engage with the question.
Granted vox might have thought: "Well, katydid just said "Virgo" and didn't explain why." Sure... but it's an online posting site. People write quick responses all the time - and they have incentive to. When people do make long, thought-out (read: non-simple-minded) posts like I often do, I'm convinced most people don't read them. These days they even seem to just kill the thread. So, instead of assuming katydid's thoughts are simple-minded, the better response in that case (humanly, but also pragmatically for you, since, again, being a jerk turns people off) is just to re-post: "Oh, why Virgo? Explain?"
Now... let me say something about why this isn't nearly as interesting and serious a question as vox seems to think it is - unless you approach it exactly the way katydid and junoisuppose have: treating the Mother Mary as a cultural archetype, and considering the zodiacal archetype that seems to correlate.
Two problems:
1) How do you propose approaching Mary as a historic figure that we can know anything about?
2) What's interesting, as students and scholars of astrology, about knowing Mary's sun sign, if it's not about archetypes?
Of problem 1) With your talk of "evidence" you seem to want to approach Mary as a documentable historic figure, a complete person we can investigate the way we might investigate Gandhi or Hitler, for example.
But where does your (or anybody's) "evidence" on Mary come from? Where it comes to historic documents, are there any that say anything substantial about Mary other than the Gospels? And the Gospels were never intended as documentation. Today, in modern times, we think of history as a quasi-scientific endeavor: documentation of actual events. But that was never the point of history prior to modern times. The Gospels, like Torah and the books of the prophets before them, were always intended as accounts of moral and theological import - not historical. (Heck, just read them. They're very different. And Jesus is quite a different figure across the four of them. If historical accuracy was the point presenting the four Gospels together would have been seen as much more of a problem than clearly it was.) The purpose of the Gospels was to paint a shifting cosmology amongst early A.D. Jews (and then, in Paul's hands, Gentiles) and to spread that shifting cosmology. The Gospels do not qualify as the kind of documentary historical documents we would look to today to establish anything about the birth, life and times of a historic figure. They cannot be evidential in the way vox seems to want them to be when you talk of "full evidence" and "prima face evidence."
Of course, given the nature of this site, the kind of evidence vox is thinking about may come from an esoteric source: psychics, channelling, etc. But if that's the case, we're even more in the realm of un-verifiability. If that's the case, me thinks box would need to do a tremendous amount of work to substantiate whatever "evidence" we're talking about, of the sort piercetheviel, for example, has done in his decade-long exploration of the chart on Jesus that he's drawn up. (See the Sabian Symbols section).
On problem 2) If we do want to treat Mary not as a basic cultural archetype but as a full, complete, complex human being for whom we can find historical evidence and who's real, individual astrology chart we might actually be able to draw up and dwell on - if we do want to say everything I just wrote about problem 1 is wrong - then what does it serve us to debate her sun sign?!? This is an amateur learning site, but not that amateur.
If, for example, the most convincing chart for Jesus ever posted on this site, out of the multitude of them - if we judge by work done by the proposer to back it up - is piercetheviel's, in that chart Jesus is an Aries sun! Now, if we look closely at that sun position, particularly considering the Sabian symbol - it's pretty convincing. But it's hardly what most people would jump to when thinking about (their cultural, archetypical impression of) Jesus. And unless we do very close work of the sort vox's original post - a simple poll - hardly invites, we won't see or learn much that's interesting in it.
In conclusion - hey, if a poster wants to make a thread asking about the sun signs of ancient figures, all power to ya. Seems like a fun thing to do. But don't scoff at people who venture an answer, as if the point of the exercise was remotely anywhere as deep and meaningful as vox is suggesting vox thinks it is. All this poll/thread can ever be is either:
1) An interesting consideration of howe the cultural archetype expressed by the ancient figure in question fits with astrological archetypes (as posters have tried to do) or
2) A bit of silly fun - not worth scoffing at anybody over.