Because this seems to be a topic that has several people hitting the panic button, here is how to pee while wearing your wedding dress:
On arrival at your venue
Open the door of the ladies' room and look in. If the floor is not clean, call the venue staff and ask to have it cleaned.
If the floor is clean, walk into the ladies' room and open the door of the handicapped stall. If the floor of the handicapped stall is not clean, call the venue staff and ask to have it cleaned.
This ensures that you will not later get the train/back of the dress dirty just by having it touch the floor.
When you need to go
Walk into the handicapped stall. Remove (don't just pull down) your underpants.
Facing the toilet, pull up the front of your dress to your waist.
Walk straight forward, so you are straddling the toilet, and sit down that way. The train/back of your dress will now be away from the toilet, so you won't get them wet or dirty.
Lean forward before peeing. That way, even if you drop the front of the dress onto the seat, you will not pee in its direction.
