Basic electronics would lead me to think somethings earthing your battery out, if they keep going flat so quickly.
Have you put a multimeter across it??
Mine's not been on a charger all winter and started once every other week (sometimes longer time periods), starting first time no probs.
I've also noticed I never need to use the choke on the Ducati, even in winter!!
You should really get in the habit of starting your bike with the lights off though.
PS: Jap bikes have the same problem so it wouldn't solve a lot.