I don't doubt that your g/f's explanation in light of her experience in the profession of wedding gown design is incorrect; however, I think it's not entirely accurate. That may very well be the reason it was started, but it sure isn't the reason women do it today.
Why do I think that? Take a poll and ask women why they think other women wear white to their weddings. Shit, look at this thread - 29 replies and you were the first one to mention this.
The commonly accepted reason for this tradition today seems to be purity. Women wear white to their weddings because they are (supposedly) virgins. Unfortunately this doesn't seem to be the case anymore, so women these days seem to wear it because "well, everyone else is doing it."
Kind of takes away from the dignity of the ceremony I think. Women shouldn't try to represent sexual purity if they're not sexually pure.
-Warik