White to symbolise purity generally dates back at least to the Romans. Queen Victoria started the trend for white wedding dresses (supposedly to support the lace industry, though I don't know if this is actually true).
I don't think it was ever an absolute rule that only virgins should wear white but, until very recently - and even now in some churches - divorcées could not remarry in church if their ex was still alive, and a second wedding was typically more low-key as divorce was regarded as shameful (my grandmother got excommunicated for being divorced!).