well it could be bad, obviously, because of germs and bacteria. but i have never heard of any dog dying from drinking toilet water, unless you count the small dogs that fell in and drown
after having kids we got used to just kepping the door shut.
As long as you do not have anything like an additive in the water to keep it clean. If it is plain water then there is really not an issue other than it grossing you out! Just remember that when he tries to kiss you!
My oldest dog drink out of the toilet and I make sure I keep them as clean as I can. I have even tried child locks and he still gets in so....... now I gave up! I even have a 2 gallon auto waterer in the master bedroom where he sleep but nothing beats a cold drink from the toilet I guess!