At a civil service it's a costume to indicate who the Bride is and leave her have her special day.
At a Christian religious ceremony it's to remind the congregation of the marriage of Jesus to his church in revelation.
Most people see it as an insult and that you are advertising to all that you are an attention seeking arsehole if you wear white to another woman's wedding.
If you don't want to wear white at your wedding then you are free to do so.