This is true, gas does repel water, however going from hot/cold situations is what creates condinsation. My old Kreidler used to build up a ton of condinsation in the fuel every fall/winter because of the extreeme weather changes from riding it around in the snow all day to parking it in the warm cellar. I had to drain my tank and clean the carb 3 times last year and eventually resorted to using a small amount of dry-gas to add to my 40:1 mix.
Gas may repel water, however it CAN mix with water, and water molocules can become suspended in the gasoline. hence why we have dry-gas (ethanol).. this bonds to the water creating a burnable alcohol which thus mixes more thuroughly with the gasoline.
This is why most moped gas-tanks rust from the BOTTOM of the tank, inside-out. The condonsation builds up, it's parked all winter, and the water trickeles to the bottom through the gas and sits on the bottom of the tank. It can only take a couple months for gasoline to lose it's octane level thus losing it's water-repelling properties. During this time the tank is extremely suseptable to rusting. Hence why a full gas-tank can still rust.
Sorry to go on a rant about this. I guess I wasnt very clear with my earlier explanation. Not like you have to worry about condinsation in the summer time anyway! lol.