Comments KnudsonRegime, convincingly enough for me:
Sewer gas is typically 20-200 ppm hydrogen sulfide and even at the upper range it's not going to do anything other than give you a sore throat and make it hard for you to smell anything else. Methane concentrations are 7-15 ppm and breathing it at those concentrations for extended periods may make you feel lightheaded and nauseous, but it shouldn't kill you. Occupational exposure of 5300 ppm over eight hours in some industries is not uncommon. The rest of sewer gas composition is regular atmosphere.
The alternative atmosphere is a structure fire will render you unconscious in 2-5 minutes. Breathing through the toilet obviously isn't ideal, but you can do it for hours with no long term effects.